slices: print modified cloned slice in ExampleClone

slices: print modified cloned slice in ExampleClone

Make it obvious that numbers wasn't modified, but clone was.

Change-Id: Id8bd4ca59b7348cd218faef61abfdf12e20c89f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/617795
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>