doc: apply sentence case to backporting-to-release-lines.md headers

PR-URL: #37617
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/backporting-to-release-lines.md

@@ -1,4 +1,4 @@
-# How to Backport a Pull Request to a Release Line
+# How to backport a pull request to a release line
 
 ## Staging branches