io: add error check to WriteString Example test
io: add error check to WriteString Example test

Change-Id: I9ce1c79e5799f205aec3a4dc02645ed26bdc3581
GitHub-Last-Rev: 59b637db0154e55ddfdd55e54b9596dc3a0ad32d
GitHub-Pull-Request: golang/go#44533
Reviewed-on: https://go-review.googlesource.com/c/go/+/295389
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Trust: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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