Fix typo on exemple of jwt usage (#245)

Author: rougeth

packages/google-auth/google/auth/jwt.py

@@ -21,7 +21,7 @@
 
 To encode a JWT use :func:`encode`::
 
-    from google.auth import crypto
+    from google.auth import crypt
     from google.auth import jwt
 
     signer = crypt.Signer(private_key)