[release-branch.go1.6] doc: update release tag in source directions to go1.6

Fixes #14371

Change-Id: I8e9e4b2b89083f5a947e791c011912fdf365a11e
Reviewed-on: https://go-review.googlesource.com/19996Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/20001Reviewed-by: Niels Widger <niels.widger@gmail.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/install-source.html

@@ -199,7 +199,7 @@ Then clone the repository and check out the latest release tag:</p>
 <pre>
 $ git clone https://go.googlesource.com/go
 $ cd go
-$ git checkout go1.5.3
+$ git checkout go1.6
 </pre>
 
 <h2 id="head">(Optional) Switch to the master branch</h2>
@@ -378,7 +378,7 @@ 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>go1.5.3</code>.
+<code>go1.6</code>.
 </p>
 
 <p>