Fix correct usage function

This fixes the push function as it currently gives the help for `run`
not `push`.

Signed-off-by: Dan Finneran <daniel.finneran@gmail.com>

src/cmd/linuxkit/push.go

@@ -23,7 +23,7 @@ func pushUsage() {
 
 func push(args []string) {
 	if len(args) < 1 {
-		runUsage()
+		pushUsage()
 		os.Exit(1)
 	}