strings: fix typo in examples

The correct word can be seen in lines 381-382.

Change-Id: If3876bd34b6433b69531763f63af88d60a0bfad0
Reviewed-on: https://go-review.googlesource.com/c/go/+/606375


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>