Fix collision layer math formula in Physics introduction (#4598)

tutorials/physics/physics_introduction.rst

@@ -167,7 +167,7 @@ would be as follows::
 
     # Decimal - Add the results of 2 to the power of (layer be enabled-1).
     # (2^(1-1)) + (2^(3-1)) + (2^(4-1)) = 1 + 4 + 8 = 13
-    pow(2, 1) + pow(2, 3) + pow(2, 4)
+    pow(2, 1-1) + pow(2, 3-1) + pow(2, 4-1)
 
 
 Area2D