fixed ParticleSystem:setSize() looking at the wrong argument for size variation

src/modules/graphics/opengl/wrap_ParticleSystem.cpp

@@ -148,7 +148,7 @@ namespace opengl
 		ParticleSystem * t = luax_checkparticlesystem(L, 1);
 		float arg1 = (float)luaL_checknumber(L, 2);
 		float arg2 = (float)luaL_optnumber(L, 3, arg1);
-		float arg3 = (float)luaL_optnumber(L, 3, 0);
+		float arg3 = (float)luaL_optnumber(L, 4, 0);
 		t->setSize(arg1, arg2, arg3);
 		return 0;
 	}