Update README.md: fix typo (#3842)

Author: thandal

nav2_mppi_controller/README.md

@@ -187,7 +187,7 @@ controller_server:
       TrajectoryVisualizer:
         trajectory_step: 5
         time_step: 3
-      AckermannConstrains:
+      AckermannConstraints:
         min_turning_r: 0.2
       critics: ["ConstraintCritic", "ObstaclesCritic", "GoalCritic", "GoalAngleCritic", "PathAlignCritic", "PathFollowCritic", "PathAngleCritic", "PreferForwardCritic"]
       ConstraintCritic: