_content/blog: fix date on tutorials-go.18.md
_content/blog: fix date on tutorials-go.18.md

Change-Id: Ie926a25fb0d239e716288c647ddb78f771b074f5
Reviewed-on: https://go-review.googlesource.com/c/website/+/378595
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Trust: DO NOT USE <b67e673702df165eba99d19477fbb63eec0d8851@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website