doc: update the URL of the latest go1.4 source snapshot

Updates golang/go#20672

Change-Id: I3c62b1606aec93e188255f1701c0af569d540016
Reviewed-on: https://go-review.googlesource.com/79276Reviewed-by: Ian Lance Taylor <iant@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-20170531.tar.gz">go1.4-bootstrap-20170531.tar.gz</a>,
+<a href="https://storage.googleapis.com/golang/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.)