_content/doc: update release branch mention to in install-source

We've updated most pages on the website to recommend using specific
release tags when building a release from source (we used to suggest
checking out the release branch, but that might give a commit that
doesn't correspond to any final release). Fix one more.

Change-Id: If4dbffacf61e3d32c8b8c0a3548ead2de6f2e76e
Reviewed-on: https://go-review.googlesource.com/c/website/+/299370
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Steve Traut <straut@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/_content/doc/install-source.html b/_content/doc/install-source.html
index 4029b17..3997694 100644
--- a/_content/doc/install-source.html
+++ b/_content/doc/install-source.html
@@ -285,7 +285,7 @@
 <p>If you intend to modify the go source code, and
 <a href="/doc/contribute.html">contribute your changes</a>
 to the project, then move your repository
-off the release branch, and onto the master (development) branch.
+off the release tag, and onto the master (development) branch.
 Otherwise, skip this step.</p>