docs: fix minor typo

[samfolo]

Thanks for contributing to chrono!

If your feature is semver-compatible, please target the main branch;
for semver-incompatible changes, please target the 0.5.x branch.

Please consider adding a test to ensure your bug fix/feature will not break in the future.

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Saw this and had to jump at the opportunity to contribute to this amazing staple project! Currently learning Rust, and hope to be more useful to efforts like these in future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.02%. Comparing base (7cdca4b) to head (51d7fa8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1659   +/-   ##
=======================================
  Coverage   91.02%   91.02%           
=======================================
  Files          37       37           
  Lines       17366    17366           
=======================================
  Hits        15808    15808           
  Misses       1558     1558           

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[djc]

Thanks!