make footprint topic relative in collision monitor while using with namespace (#606)

Author: KishanSawant

turtlebot4_navigation/config/nav2.yaml

@@ -309,7 +309,7 @@ collision_monitor:
     FootprintApproach:
       type: "polygon"
       action_type: "approach"
-      footprint_topic: "/local_costmap/published_footprint"
+      footprint_topic: "local_costmap/published_footprint"
       time_before_collision: 1.2
       simulation_time_step: 0.1
       min_points: 6
@@ -368,4 +368,4 @@ docking_server:
       k_phi: 3.0
       k_delta: 2.0
       v_linear_min: 0.15
-      v_linear_max: 0.15
+      v_linear_max: 0.15