[release-branch.go1.4] doc: update source install instruction to use tag 'go1.4.1'

Change-Id: I12e531fc0d92d3b6fc7ec2bbd8c029f63f55fbe1
Reviewed-on: https://go-review.googlesource.com/2798
Reviewed-by: Russ Cox <rsc@golang.org>
1 file changed