_content/doc/tutorial: alias go to go1.18beta1 in generics.md
_content/doc/tutorial: alias go to go1.18beta1 in generics.md

My memory and documentation I just looked up suggests the alias name
comes first, and the value second. Thanks to Hana for spotting this.

Change-Id: I981e49c5a9231c258606a8a7914ac9857923d903
Reviewed-on: https://go-review.googlesource.com/c/website/+/372055
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>

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