Update the go/types guide to have a link to the new 1.18 API guide, for
Also fix some links to pkg.go.dev and golang.org/x that were amended in
the autogenerated README.md rather than the original source.
Trust: Robert Findley <firstname.lastname@example.org>
Run-TryBot: Robert Findley <email@example.com>
Reviewed-by: Robert Griesemer <firstname.lastname@example.org>
TryBot-Result: Gopher Robot <email@example.com>