simplication of lattice_generator.py, fixes #3858 (#3859)

* simplification of equation to compute the max_value/outer edge of the lattice based on number of headings.

nav2_smac_planner/lattice_primitives/lattice_generator.py

@@ -119,7 +119,7 @@ def _get_heading_discretization(self, number_of_headings: int) -> list:
             A list of headings in radians
 
         """
-        max_val = int((((number_of_headings + 4) / 4) - 1) / 2)
+        max_val = int(number_of_headings / 8)
 
         outer_edge_x = []
         outer_edge_y = []