doc: update bootstrap archive URL

This includes the patch for systems that build PIE executables by
defaul

Updates #20276.

Change-Id: Iecf8dfcf11bc18d397b8075559c37e3610f825cb
Reviewed-on: https://go-review.googlesource.com/44470Reviewed-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-20161024.tar.gz">go1.4-bootstrap-20161024.tar.gz</a>,
+<a href="https://storage.googleapis.com/golang/go1.4-bootstrap-20170531.tar.gz">go1.4-bootstrap-20170531.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.)