cmd/go: `go help build` mentions -mod=release, which is not supported

Using go 1.11.

```
$ go help build | grep -A2 -- -mod
	-mod mode
		module download mode to use: readonly, release, or vendor.
		See 'go help modules' for more.

$ go test -mod=release
-mod=release not supported (can be '', 'readonly', or 'vendor')

$ go build -mod=release
-mod=release not supported (can be '', 'readonly', or 'vendor')
```