go/parser: use valid position when reporting an error (copy-paste bug)

This is a follow-up on https://golang.org/cl/202581.

Updates #33649.

Change-Id: Ib078fed983792c5493bdbed6d33e21b86856894a
Reviewed-on: https://go-review.googlesource.com/c/go/+/204041
Run-TryBot: Robert Griesemer <gri@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

src/go/parser/parser.go

@@ -403,7 +403,7 @@ func (p *parser) expect2(tok token.Token) (pos token.Pos) {
 	if p.tok == tok {
 		pos = p.pos
 	} else {
-		p.errorExpected(pos, "'"+tok.String()+"'")
+		p.errorExpected(p.pos, "'"+tok.String()+"'")
 	}
 	p.next() // make progress
 	return