flag: fix error in documentation example.

Fixes #1615.
This time for sure.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/4275079

src/pkg/flag/flag.go

@@ -56,7 +56,7 @@
 
 		flag.Bool(...)  // global options
 		flag.Parse()  // parse leading command
-		subcmd := flag.Arg[0]
+		subcmd := flag.Arg(0)
 		switch subcmd {
 			// add per-subcommand options
 		}