Update hpke/xkem.go

Co-authored-by: Armando Faz <armfazh@users.noreply.github.com>

hpke/xkem.go

@@ -87,7 +87,7 @@ func (x xKEM) UnmarshalBinaryPrivateKey(data []byte) (kem.PrivateKey, error) {
 	sk := &xKEMPrivKey{x, make([]byte, l), nil}
 	copy(sk.priv, data[:l])
 	if !sk.validate() {
-		return nil, ErrInvalidKEMPublicKey
+		return nil, ErrInvalidKEMPrivateKey
 	}
 	return sk, nil
 }