✏️ Fix typo in turtlebot3_mission_controller description

Signed-off-by: Luiz Carlos Cosmi Filho <[email protected]>

Author: luizcarloscf

README.md

@@ -8,7 +8,7 @@ The package's goal is to use the robot [turtlebot3](https://github.com/ROBOTIS-G
 
 * [turtlebot3_explorer](turtlebot3_mapper/turtlebot3_explorer/turtlebot3_explorer.py): it subscribes to receive messages from the laser sensor and publishes velocity commands. If any obstacle is detected in front of the robot, it then rotates until it finds a free path again. Also has a service that allows to enable or disable this behavior.
 
-* [turtlebot3_mission_controller](turtlebot3_mapper/turtlebot3_mission_controller/turtlebot3_mission_controller.py): the main service of this node is an action server responsible for determining when the exploration starts and ends. To do so, it uses the concept of fronteirs (intersection between unknown regions and free regions). Through the objective sent to the action server, where a user can specify the number of remaining fronteir points, it will enable the `turtlebot3_explorer` to explore the enviromment until it reaches the specified numbers of frontiers points in the occupancy grid.
+* [turtlebot3_mission_controller](turtlebot3_mapper/turtlebot3_mission_controller/turtlebot3_mission_controller.py): the main service of this node is an action server responsible for determining when the exploration starts and ends. To do so, it uses the concept of fronteirs (intersection between unknown regions and free regions). Through the goal sent to the action server, where a user can specify the number of remaining fronteir points, it will enable the `turtlebot3_explorer` to explore the enviromment until it reaches the specified numbers of frontiers points in the occupancy grid.
 
 * [turtlebot3_mission_client](turtlebot3_mapper/turtlebot3_mission_client/turtlebot3_mission_client.py): action client responsible for sending the exploration task to the action server and saving the results to a file.