src/README.vendor: s/latest/master/

src/README.vendor: s/latest/master/

Update the README to specify the module query "@master" instead of
"@latest".

Vendoring the highest tagged version is unlikely to be right. Usually
one wants to vendor the module at HEAD.

Change-Id: Id00d23523a13fd3dcd73d6eacefdf50bcdbfa26e
Reviewed-on: https://go-review.googlesource.com/c/go/+/508823
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>