doc: remove mention of %HOME% from installation instructions

Fixes #15598

Change-Id: I4cfb8799dab0e9e34cae2e61839911fd65e4cfa3
Reviewed-on: https://go-review.googlesource.com/23004Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install.html

@@ -221,7 +221,7 @@ and building a simple program, as follows.
 <p>
 Create a directory to contain your <a href="code.html#Workspaces">workspace</a>,
 <code class="testUnix">$HOME/work</code>
-<code class="testWindows" style="display: none">%HOME%\work</code>
+<code class="testWindows" style="display: none">C:\work</code>
 for example, and set the <code>GOPATH</code> environment
 variable to point to that location.
 </p>
@@ -231,7 +231,7 @@ $ <b>export GOPATH=$HOME/work</b>
 </pre>
 
 <pre class="testWindows" style="display: none">
-C:\&gt; <b>set GOPATH=%HOME%\work</b>
+C:\&gt; <b>set GOPATH=C:\work</b>
 </pre>
 
 <p>