doc/go1.18: correct "go build -asan" HTML tag

The tag was "go-mod-vendor", which doesn't match the content.

Also move that section later, so "go mod" sections stay together.

For #47694.

Change-Id: Id4fa7ee0768682a9aadfeb1b2f1d723e7521896b
Reviewed-on: https://go-review.googlesource.com/c/go/+/387354


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>