Fix apparent typo in call to jwt.decode()

Used `algorithm='RS256'` as is done with jwt.encode() but
in decode() it's plural `algorithms` and it sets which algorithms
are valid -- so it needs to be a list and not a string.

social_core/backends/apple.py

@@ -125,7 +125,7 @@ def decode_id_token(self, id_token):
                 id_token,
                 key=public_key,
                 audience=self.get_audience(),
-                algorithm='RS256',
+                algorithms=['RS256'],
             )
         except PyJWTError:
             raise AuthFailed(self, 'Token validation failed')