Bug 1603707- Edited the doc for adding a supplementary instruction. r=sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D57098

--HG--
extra : moz-landing-system : lando

tools/docs/docs/contribute/how_to_contribute_firefox.rst

@@ -170,6 +170,8 @@ Run:
 
     $ hg commit --amend <the modified file>
 
+After amending the patch, you will need to submit it using moz-phab again.
+
 If you wrote many changes, you can squash or edit commits with the
 command: