[release-branch.go1.4] doc: update source install instruction to use tag 'go1.4.1'

Change-Id: I12e531fc0d92d3b6fc7ec2bbd8c029f63f55fbe1
Reviewed-on: https://go-review.googlesource.com/2798Reviewed-by: Russ Cox <rsc@golang.org>

doc/install-source.html

@@ -109,7 +109,7 @@ 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 go1.4.1
 </pre>
 
 <h2 id="head">(Optional) Switch to the master branch</h2>