contribute.html: do not fill in the reviewer field

The golang-dev mailing list is added automatically.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5482060

doc/contribute.html

@@ -210,10 +210,10 @@ to this change, and the <code>CC</code> line lists people to
 notify about the change.
 These can be code review nicknames or arbitrary email addresses.
 Unless explicitly told otherwise, such as in the discussion leading
-up to sending in the change list, set the
-reviewer field to the
+up to sending in the change list, leave the reviewer field blank.
+This means that the
 <a href="http://groups.google.com/group/golang-dev">golang-dev@googlegroups.com</a>
-mailing list.
+mailing list will be used as the reviewer.
 </p>
 
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