Update __init__.py

Author: jamesshannon

python/py_vapid/__init__.py

@@ -41,7 +41,7 @@ def __init__(self, private_key=None):
         """Initialize VAPID with an optional private key.
 
         :param private_key: A private key object
-        :type private_key: ecdsa.SigningKey
+        :type private_key: ec.EllipticCurvePrivateKey
 
         """
         self.private_key = private_key