crypto/x509: fix copy/paste-o in error message

Fixes #15371

Change-Id: Iff8d36e1bd9b5641f6b577a30ac6e967f973c939
Reviewed-on: https://go-review.googlesource.com/22240Reviewed-by: Andrew Gerrand <adg@golang.org>

src/crypto/x509/x509.go

@@ -1133,7 +1133,7 @@ func parseCertificate(in *certificate) (*Certificate, error) {
 				if rest, err := asn1.Unmarshal(e.Value, &keyid); err != nil {
 					return nil, err
 				} else if len(rest) != 0 {
-					return nil, errors.New("x509: trailing data after X.509 authority key-id")
+					return nil, errors.New("x509: trailing data after X.509 key-id")
 				}
 				out.SubjectKeyId = keyid