doc: amend installation instructions

Fixes golang#8674.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/139320044

doc/install.html

@@ -224,19 +224,12 @@ <h2 id="gopath">Set up your work environment</h2>
 
 <p>
 You're almost done.
-You just need to do a little more setup.
+You just need to set up your environment.
 </p>
 
 <p>
-<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>
-</p>
-
-<p>
-The <a href="/doc/code.html">How to Write Go Code</a> document
-provides <b>essential setup instructions</b> for using the Go tools.
+Read the <a href="/doc/code.html">How to Write Go Code</a> document,
+which provides <b>essential setup instructions</b> for using the Go tools.
 </p>