doc: fixes to contributor docs

Change-Id: Ib31fb1f8239d8113728bc4c6e411416721571341
Reviewed-on: https://go-review.googlesource.com/1743Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -139,7 +139,7 @@ Make sure <code>git-codereview</code> is installed in your shell path, so that t
 </p>
 
 <pre>
-$ git review help
+$ git codereview help
 </pre>
 
 <p>
@@ -165,7 +165,7 @@ by typing, for instance,
 </p>
 
 <pre>
-$ git review sync
+$ git codereview sync
 </pre>
 
 <p>
@@ -204,7 +204,7 @@ To install them, copy this text into your Git configuration file
 <p>After installing the <code>git-codereview</code> command, you can run</p>
 
 <pre>
-$ git review help
+$ git codereview help
 </pre>
 
 <p>