Fix a typo in credentials.py.

Author: craigcitro

google/auth/credentials.py

@@ -53,7 +53,7 @@ def __init__(self):
     def expired(self):
         """Checks if the credentials are expired.
 
-        Note that credentials can be invalid but not expired becaue Credentials
+        Note that credentials can be invalid but not expired because Credentials
         with :attr:`expiry` set to None is considered to never expire.
         """
         if not self.expiry: