Fix typograpical errors (#2438)

src/transformers/modeling_tf_bert.py

@@ -576,7 +576,7 @@ class TFBertMainLayer(tf.keras.layers.Layer):
 
 class TFBertPreTrainedModel(TFPreTrainedModel):
     """ An abstract class to handle weights initialization and
-        a simple interface for dowloading and loading pretrained models.
+        a simple interface for downloading and loading pretrained models.
     """
 
     config_class = BertConfig