doc/contribute: try to make it clearer that you send a review with hg mail, not hg submit.

R=r
https://golang.org/cl/171046

doc/contribute.html

@@ -409,10 +409,10 @@ The reviewer approves the change by replying with a mail that says
 <code>LGTM</code>: looks good to me.
 </p>
 
-<h3>Submit the change</h3>
+<h3>Submit the change after the review</h3>
 
 <p>
-Once the code has been <code>LGTM</code>'ed, it is time to submit
+After the code has been <code>LGTM</code>'ed, it is time to submit
 it to the Mercurial repository.
 If you are a committer, you can run:
 </p>