cmd/go: fix bug help message

The bug subcommand opens up the browser instead of printing information.
Fixes help message to reflect that.

Fixes #18630.

Change-Id: I660c94bc65ef1994292cfd72d08a544699545701
Reviewed-on: https://go-review.googlesource.com/35150Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/bug.go

@@ -20,11 +20,10 @@ import (
 var cmdBug = &Command{
 	Run:       runBug,
 	UsageLine: "bug",
-	Short:     "print information for bug reports",
+	Short:     "start a bug report",
 	Long: `
-Bug prints information that helps file effective bug reports.
-
-Bugs may be reported at https://golang.org/issue/new.
+Bug opens the default browser and starts a new bug report.
+The report includes useful system information.
 	`,
 }