contribute.html: suggest hg mail instead of hg upload

R=r
CC=golang-dev
https://golang.org/cl/186145

doc/contribute.html

@@ -416,15 +416,15 @@ When you have revised the code and are ready for another round of review, run
 </p>
 
 <pre>
-$ hg upload 99999
+$ hg mail 99999
 </pre>
 
-<p>to upload the latest copy.
+<p>again to upload the latest copy and send mail asking the reviewers to please take another look
+(<code>PTAL</code>).
 You might also visit the code review web page and reply to the comments,
 letting the reviewer know that you've addressed them or explain why you
 haven't.  When you're done replying, click &ldquo;Publish and Mail comments&rdquo;
 to send the line-by-line replies and any other comments.
-A common acronym in such mails is <code>PTAL</code>: please take another look.
 </p>
 <p>
 The reviewer can comment on the new copy, and the process repeats.