[Doc] Fix mistakes in docs for Trainer checkpointing backends

[kurtamohler]

Description

Fix documentation for Trainer checkpointing, which mistakenly said that torchsnapshot is the default backend, when torch is actually the default, as seen here.

Also fix incorrect syntax for the hyperlink to the torchsnapshot repo.

Motivation and Context

close #2075

• Someone has raised an issue to propose this change (required for new features and bug fixes)

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

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• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2285

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❌ 1 New Failure, 8 Pending, 16 Unrelated Failures

As of commit 6cf8ee0 with merge base 3566388 ():

NEW FAILURE - The following job has failed:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
  Workflow failed! Resource not accessible by integration

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Examples Tests on Linux / tests (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Generate documentation / build-docs (3.9, 12.1) / linux-job (gh) (similar failure)
• Habitat Tests on Linux / tests (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Libs Tests on Linux / unittests-gym (3.9, 12.1) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Libs Tests on Linux / unittests-sklearn (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Lint / c-source / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Lint / python-source-and-configs / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• RLHF Tests on Linux / unittests (3.9, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-cpu (3.10) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-cpu (3.11) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-cpu (3.8) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Unit-tests on Linux / tests-cpu (3.9) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-gpu (3.10, 12.1) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Unit-tests on Linux / tests-stable-gpu (3.10, 11.8) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[vmoens]

LGTM thanks!