Make footprint topic relative in collision monitor to allow using with namespace

[KishanSawant]

Description

Removed a leading forward slash from the "local_costmap/published_footprint" topic under collision_monitor to make it relative.
This change resolves the warning encountered when launching the TurtleBot with a namespace:
[collision_monitor]: [FootprintApproach]: Polygon shape is not set yet

The issue was due to the collision monitor not finding the footprint topic when launched within a namespace.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• When launching navigation with a namespace, the warning no longer appears.
• The setup also works as expected without a namespace.
• After launching the navigation stack and setting a goal pose in RViz2 using the 2D Goal Pose, the robot successfully navigates toward the goal without any warnings.

# Run this command
ros2 launch turtlebot4_gz_bringup turtlebot4_gz.launch.py namespace:=robot1 use_sim_time:=true
ros2 launch turtlebot4_navigation localization.launch.py namespace:=robot1 map:=<path_to_map_config_file> use_sim_time:=true
ros2 launch turtlebot4_naviagtion nav2.launch.py namespace:=robot1 use_sim_time:=true

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation