doc/install.html: fix duplicate id= tag

LGTM=minux.ma
R=adg, minux.ma
CC=golang-codereviews
https://golang.org/cl/95540045

doc/install.html

@@ -228,7 +228,7 @@ You just need to do a little more setup.
 </p>
 
 <p>
-<a href="/doc/code.html" class="download" id="start">
+<a href="/doc/code.html" class="download" id="writing">
 <span class="big">How to Write Go Code</span>
 <span class="desc">Learn how to set up and use the Go tools</span>
 </a>