cmd/go: don't mention -mod=release

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

Fixes #27354.

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.