cmd/go: improve packages help description

Clarify that 'action' is not the valid 'go' command.

Change-Id: I0a77722c46a3dc56f81c5e6e177e0c73bc60adc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/482455
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: alexandear

src/cmd/go/alldocs.go

@@ -34,7 +34,7 @@ var HelpPackages = &base.Command{
 	Long: `
 Many commands apply to a set of packages:
 
-	go action [packages]
+	go <action> [packages]
 
 Usually, [packages] is a list of import paths.