Fix typo on exemple of jwt usage (#245)

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 6957374..ef23db2 100644
--- a/google/auth/jwt.py
+++ b/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)