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/62910Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>

src/cmd/go/internal/get/vcs.go

@@ -940,7 +940,7 @@ var vcsPaths = []*vcsPath{
 	// chiselapp.com for fossil
 	{
 		prefix: "chiselapp.com",
-		re:     `^(?P<root>chiselapp\.com/user/[A-Za-z0-9]+/repository/[A-za-z0-9_.\-]+)$`,
+		re:     `^(?P<root>chiselapp\.com/user/[A-Za-z0-9]+/repository/[A-Za-z0-9_.\-]+)$`,
 		vcs:    "fossil",
 		repo:   "https://{root}",
 	},