_content/doc/go1.18: update expected release date

Go 1.18 is now expected to be released in March 2022.

For golang/go#44513.
Fixes golang/go#51505.

Change-Id: I4f744dc96c6ef5150d7475b51759d5fc5b13633b
Reviewed-on: https://go-review.googlesource.com/c/website/+/390374
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: DO NOT USE <iant@google.com>

_content/doc/go1.18.html

@@ -19,7 +19,7 @@ <h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.18</h2>
 <p>
   <strong>
     Go 1.18 is not yet released. These are work-in-progress
-    release notes. Go 1.18 is expected to be released in February 2022.
+    release notes. Go 1.18 is expected to be released in March 2022.
   </strong>
 </p>