Update modified default_bt_xml_filename parameter (#1797)

Parameter ``bt_xml_filename`` has changed to ``default_bt_xml_filename``

doc/parameters/param_list.md

@@ -4,7 +4,7 @@
 
 | Parameter | Default | Description |
 | ----------| --------| ------------|
-| bt_xml_filename | N/A | path to behavior tree XML description |
+| default_bt_xml_filename | N/A | path to the default behavior tree XML description |
 | plugin_lib_names | ["nav2_compute_path_to_pose_action_bt_node", "nav2_follow_path_action_bt_node", "nav2_back_up_action_bt_node", "nav2_spin_action_bt_node", "nav2_wait_action_bt_node", "nav2_clear_costmap_service_bt_node", "nav2_is_stuck_condition_bt_node", "nav2_goal_reached_condition_bt_node", "nav2_initial_pose_received_condition_bt_node", "nav2_goal_updated_condition_bt_node", "nav2_reinitialize_global_localization_service_bt_node", "nav2_rate_controller_bt_node", "nav2_distance_controller_bt_node", "nav2_recovery_node_bt_node", "nav2_pipeline_sequence_bt_node", "nav2_round_robin_node_bt_node", "nav2_transform_available_condition_bt_node"] | list of behavior tree node shared libraries |
 | transform_tolerance | 0.1 | TF transform tolerance |
 | global_frame | "map" | Reference frame |
@@ -581,4 +581,4 @@ Namespace: /parent_ns/local_ns
 
 | Input Port | Default | Description |
 | ---------- | ------- | ----------- |
-| hz | 10.0 | Rate to throttle |
+| hz | 10.0 | Rate to throttle |