flag: add FlagSet example
Add an example for using FlagSet.
Fixes #36307
Change-Id: I0bf5805bd836a4f2e9632aafe22dc3eeb1164dcd
GitHub-Last-Rev:
79e53040cbc2658a71385b644107f8ea54132c99
GitHub-Pull-Request: golang/go#64443
Reviewed-on: https://go-review.googlesource.com/c/go/+/545736
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Christopher Taylor <ccmtaylor@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Rob Pike <r@golang.org>