doc: fix typographical error in fixtures README

[Trott]

Fixing a typo. Although it looks like this file comes from https://github.com/tc39/source-map-tests. Did we ever get tooling in place to sync from there?

[Trott]

(Mostly opening this to test the GitHub bot is still working, since I merged a dependency update. As long as the bot labels it, it's fine to close it. I'm not invested in it. Although we should definitely get that update-tooling together. Or if it's already in place, run it.)

[Trott]

Closing in favor of #60978

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.53%. Comparing base (37d9cfc) to head (a9dbaae).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@ ## main #60977 +/- ## ========================================== + Coverage 88.51% 88.53% +0.01%  ========================================== Files 703 703 Lines 208430 208430 Branches 40198 40199 +1 ========================================== + Hits 184491 184531 +40  + Misses 15951 15898 -53  - Partials 7988 8001 +13 

see 44 files with indirect coverage changes

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