doc: consistently link to the SettingGOPATH page

Change-Id: I4fdd81aa7c9b180cb72ec4af3e7d9d803c99ecac
Reviewed-on: https://go-review.googlesource.com/36033
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install.html

@@ -222,8 +222,7 @@ <h2 id="testing">Test your installation</h2>
 Create your <a href="code.html#Workspaces">workspace</a> directory,
 <code class="testUnix">$HOME/go</code><code class="testWindows">%USERPROFILE%\go</code>.
 (If you'd like to use a different directory,
-you will need to set the <code>GOPATH</code> environment variable;
-see <a href="code.html#Workspaces">How to Write Go Code</a> for details.)
+you will need to <a href="https://golang.org/wiki/SettingGOPATH">set the <code>GOPATH</code> environment variable</a>.)
 </p>
 
 <p>