exp/ebnf: manual fixup for error

(The definition of ErrorList is in another file, so gofix
has no hope of getting this right.)

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5330043

src/pkg/exp/ebnf/parser.go

@@ -187,5 +187,5 @@ func (p *parser) parse(filename string, src io.Reader) Grammar {
 func Parse(filename string, src io.Reader) (Grammar, os.Error) {
 	var p parser
 	grammar := p.parse(filename, src)
-	return grammar, p.errors.Error()
+	return grammar, p.errors.Err()
 }