doc: rename @go_nuts to @golang

Yay!

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/14256043

doc/help.html

@@ -37,7 +37,7 @@ Go IRC channel.</p>
 <h3 id="plus"><a href="https://plus.google.com/101406623878176903605/posts">The Go Programming Language at Google+</a></h3>
 <p>The Go project's Google+ page.</p>
 
-<h3 id="twitter"><a href="http://twitter.com/go_nuts">@go_nuts at Twitter</a></h3>
+<h3 id="twitter"><a href="http://twitter.com/golang">@golang at Twitter</a></h3>
 <p>The Go project's official Twitter account.</p>
 <p>Tweeting your about problem with the <code>#golang</code> hashtag usually
 generates some helpful responses.</p>