doc: fix typo in contribute.html

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5795050

doc/contribute.html

@@ -404,7 +404,7 @@ $ hg mail -r golang-dev@googlegroups.com --cc math-nuts@swtch.com 99999
 
 <p>
 Running <code>hg mail</code> will send an email to you and the reviewers
-asking them to visit the issue's URL and make coments on the change.
+asking them to visit the issue's URL and make comments on the change.
 When done, the reviewer clicks &ldquo;Publish and Mail comments&rdquo;
 to send comments back.
 </p>