docs: fix typos in extension readme (#2227)

## Description

Small typo fix.

## Related Issue

N/A

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Other (security config, docs update, etc)

## Checklist before merging

- [x] Test, docs, adr added or updated as needed
- [x] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/CONTRIBUTING.md#developer-workflow)
followed

src/extensions/README.md

@@ -1 +1 @@
-This folder holds the source code for the extensions unitl the WASM extension support is added. Types should be stored in `/src/types/extensions/<name>.go` and the extension code should be stored in `/src/extensions/<name>/*`.
+This folder holds the source code for the extensions until WASM extension support is added. Types should be stored in `/src/types/extensions/<name>.go` and the extension code should be stored in `/src/extensions/<name>/*`.