fix typo

Author: baggepinnen

docs/src/trajectory_planning.md

@@ -1,4 +1,4 @@
-# Trajectory_planning
+# Trajectory planning
 
 Two methods of planning trajectories are available
 - [`point_to_point`](@ref): Generate a minimum-time point-to-point trajectory with specified start and endpoints, not exceeding specified speed and acceleration limits.
@@ -51,4 +51,4 @@ Pages   = ["trajectory_planning.md"]
 ```@autodocs
 Modules = [Multibody]
 Pages   = ["path_planning.jl", "ptp.jl"]
-```
+```