docs(contributing): correct push -f command

`git push -f` needs branch specification

In all cases, please consult the git manpages before consulting angular's contributing guide
when you need help with git, thx ^^

Closes angular#10356

CONTRIBUTING.md

@@ -127,7 +127,7 @@ Before you submit your pull request consider the following guidelines:
 
     ```shell
     git rebase master -i
-    git push -f
+    git push origin my-fix-branch -f
     ```
 
 That's it! Thank you for your contribution!