Fix include for box2d

Follow box2d repo convention.

Missiles.h

@@ -6,7 +6,7 @@
 # include <vector>
 
 // bBox2D
-# include <Box2D/Box2D.h>
+# include <box2d/box2d.h>
 
 // Identification
 enum Identity { LAND = 0, PLANE = 1, INERT = 2, ANG = 3, ARROW = 4, CHARGE = 5 };
@@ -26,4 +26,4 @@ void createInert( b2Vec2 from, b2Body* target, b2World &world, Identity missile
 
 b2Vec2 anticipate( b2Body* missile, b2Body* target );
 
-# endif // MISSILES_H
+# endif // MISSILES_H

main.cpp

@@ -7,7 +7,7 @@
 # include <SDL/SDL_gfxPrimitives.h>
 
 // bBox2D
-# include <Box2D/Box2D.h>
+# include <box2d/box2d.h>
 
 // Missiles
 # define MULTI 200.0f