doc: apply consistent punctuation to header contributing guide

PR-URL: #38047
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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.