doc: apply consistent punctuation to header contributing guide

PR-URL: #38047
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Juan José Arboleda <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: marsonya

Diff for: doc/guides/contributing/pull-requests.md

@@ -373,7 +373,7 @@ does not land, the submitters should come away from the experience feeling like
 their effort was not wasted or unappreciated. Every Pull Request from a new
 contributor is an opportunity to grow the community.
 
-### Review a bit at a time.
+### Review a bit at a time
 
 Do not overwhelm new contributors.