doc: remove the suffix number of the anchor link

Remove the number '11' as the suffix anchor id for the 'Developer's
Certificate of Origin 1.1'.

PR-URL: #29468
Refs: #6257
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 * [Code of Conduct](#code-of-conduct)
 * [Issues](#issues)
 * [Pull Requests](#pull-requests)
-* [Developer's Certificate of Origin 1.1](#developers-certificate-of-origin-11)
+* [Developer's Certificate of Origin 1.1](#developers-certificate-of-origin)
 
 ## [Code of Conduct](./doc/guides/contributing/coc.md)