[release-branch.go1.7] doc: update version tag in source install instructions

Change-Id: Id83e0371b7232b01be83640ef1e47f9026cf2a23
Reviewed-on: https://go-review.googlesource.com/27055Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/27056Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/install-source.html

@@ -203,7 +203,7 @@ To build without <code>cgo</code>, set the environment variable
 Change to the directory that will be its parent
 and make sure the <code>go</code> directory does not exist.
 Then clone the repository and check out the latest release tag
-(<code class="versionTag">go1.6</code>, for example):</p>
+(<code class="versionTag">go1.7</code>, for example):</p>
 
 <pre>
 $ git clone https://go.googlesource.com/go
@@ -391,7 +391,7 @@ 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 class="versionTag">go1.6</code>.
+<code class="versionTag">go1.7</code>.
 </p>
 
 <p>