docs: fix Markdown link on Contributing page

[jenstroeger]

Description

Fix a Markdown link on the Contributing page here (point 7).

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

n/a

Steps to Test This Pull Request

n/a

Additional context

n/a

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/bump.py	100.00% <100.00%> (ø)
commitizen/changelog.py	99.39% <100.00%> (-0.11%)	⬇️
commitizen/changelog_formats/asciidoc.py	100.00% <100.00%> (ø)
commitizen/changelog_formats/base.py	100.00% <100.00%> (ø)
commitizen/changelog_formats/markdown.py	100.00% <100.00%> (ø)
commitizen/changelog_formats/restructuredtext.py	100.00% <100.00%> (ø)
commitizen/changelog_formats/textile.py	100.00% <100.00%> (ø)
commitizen/commands/bump.py	97.72% <100.00%> (+0.08%)	⬆️
commitizen/commands/changelog.py	99.07% <100.00%> (+0.13%)	⬆️
... and 25 more

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