design: fix a typo in the go/ast type parameters document
design: fix a typo in the go/ast type parameters document

invalid or invalid -> valid or invalid, as pointed out in
https://github.com/golang/go/issues/47781#issuecomment-901793448

(thanks)

Change-Id: I203a06a3280bb6a20ac87f78ddf49990b1812331
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/343549
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/proposal,golang/proposal