[release-branch.go1.11] cmd/go: don't mention -mod=release

The -mod=release flag is not supported, so this appears to be a
documentation mistake.

Updates #27354.
Fixes #27398.

Change-Id: I895e8d5b4918adcb1f605361773173f312fa7b65
GitHub-Last-Rev: 42bfe0c
GitHub-Pull-Request: #27358
Reviewed-on: https://go-review.googlesource.com/132116
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
(cherry picked from commit 014901c)
Reviewed-on: https://go-review.googlesource.com/c/139421
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: mark-rushakoff

src/cmd/go/alldocs.go

@@ -99,7 +99,7 @@ and test commands:
 		link against shared libraries previously created with
 		-buildmode=shared.
 	-mod mode
-		module download mode to use: readonly, release, or vendor.
+		module download mode to use: readonly or vendor.
 		See 'go help modules' for more.
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.