docs: Fix invalid PR title in examples of valid PR titles (facebookincubator#11577)

Summary:
Pull Request resolved: facebookincubator#11577

"feature" is not a valid type keyword, only "feat" is ("feature" won't match the regex)

Reviewed By: pedroerp

Differential Revision: D66134945

fbshipit-source-id: d86da16aa825e99446e65099422bcf185111ff77

Author: Kevin Wilfong

CONTRIBUTING.md

@@ -164,7 +164,7 @@ where:
 
 Examples of PR titles are:
 
-* feature(type): Add IPPREFIX
+* feat(type): Add IPPREFIX
 * fix: Prevent unnecessary flatmap to map conversion
 * refactor(vector): Use 'if constexpr' for Buffer::is_pod_like_v\<T\>