Document the type of unverified JWS claims

So far the docstring for `jws.get_unverified_claims()` stated that the
method returned a dict, while in fact it returns a string.

This commit adjusts the documentation to reflect the actual behaviour.

jose/jws.py

@@ -118,7 +118,7 @@ def get_unverified_claims(token):
  token (str): A signed JWS to decode the headers from.
 
  Returns:
- dict: The dict representation of the token claims.
+ str: The str representation of the token claims.
 
  Raises:
  JWSError: If there is an exception decoding the token.