cmd/go: fix regexp

cmd/go: fix regexp

The regular expression "A-za-z" is most likely a typo and
the intent seems to be "A-Za-z" instead.

Using "A-z" matches certain characters like: [\]^_`

Updates #10010

Change-Id: If2d064c56ef613f2e46285d8d4e5998e83aed43a
Reviewed-on: https://go-review.googlesource.com/62910
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>