doc: update URL of the go1.4 source snapshot to use dl.google.com

Updates golang/go#20672

Change-Id: I88a1d8693ef9d1e4758719603ce1f3c3f6b920bc
Reviewed-on: https://go-review.googlesource.com/79936Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -143,7 +143,7 @@ packaged Go distribution.
 <p>
 To build a bootstrap tool chain from source, use
 either the git branch <code>release-branch.go1.4</code> or
-<a href="https://storage.googleapis.com/golang/go1.4-bootstrap-20171003.tar.gz">go1.4-bootstrap-20171003.tar.gz</a>,
+<a href="https://dl.google.com/go/go1.4-bootstrap-20171003.tar.gz">go1.4-bootstrap-20171003.tar.gz</a>,
 which contains the Go 1.4 source code plus accumulated fixes
 to keep the tools running on newer operating systems.
 (Go 1.4 was the last distribution in which the tool chain was written in C.)