Fix incorrect doxygen comment (#4741)

Signed-off-by: Ryan Friedman <25047695+Ryanf55@users.noreply.github.com>

Author: Ryanf55

nav2_costmap_2d/include/nav2_costmap_2d/costmap_2d.hpp

@@ -150,8 +150,7 @@ class Costmap2D
 
   /**
    * @brief  Get the cost of a cell in the costmap
-   * @param mx The x coordinate of the cell
-   * @param my The y coordinate of the cell
+   * @param index The cell index
    * @return The cost of the cell
    */
   unsigned char getCost(unsigned int index) const;