Fix typograpical errors (#2438)

templates/adding_a_new_model/modeling_tf_xxx.py

@@ -176,7 +176,7 @@ class TFXxxMainLayer(tf.keras.layers.Layer):
 ####################################################
 class TFXxxPreTrainedModel(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 = XxxConfig

templates/adding_a_new_model/modeling_xxx.py

@@ -173,7 +173,7 @@ XxxPooler = nn.Module
 
 class XxxPreTrainedModel(PreTrainedModel):
     """ 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 = XxxConfig