Fix typo (keras-team#7191)

Author: nzw0301

keras/models.py

@@ -1067,7 +1067,7 @@ def fit_generator(self, generator,
                 for the class.
             max_queue_size: Maximum size for the generator queue
             workers: Maximum number of processes to spin up
-            use_multiprocessing: Ff True, use process based threading.
+            use_multiprocessing: if True, use process based threading.
                 Note that because
                 this implementation relies on multiprocessing,
                 you should not pass