doc: update install from source instructions for go1.5

Fixes #11983

Change-Id: I5ee930314a43356f5be31d758d90d7ddcafc7b37
Reviewed-on: https://go-review.googlesource.com/12908Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/install-source.html

@@ -167,7 +167,7 @@ Then clone the repository and check out the latest release tag:</p>
 <pre>
 $ git clone https://go.googlesource.com/go
 $ cd go
-$ git checkout go1.4
+$ git checkout go1.5
 </pre>
 
 <h2 id="head">(Optional) Switch to the master branch</h2>
@@ -346,7 +346,7 @@ New releases are announced on the
 <a href="//groups.google.com/group/golang-announce">golang-announce</a>
 mailing list.
 Each announcement mentions the latest release tag, for instance,
-<code>go1.4</code>.
+<code>go1.5</code>.
 </p>
 
 <p>