fixed documentation and function name

[KnightArthurRen]

There are some documentations regarding world plugin that are outdated and misleading, this is just a fix on these documentations

[josephduchesne]

Errors << flatland_plugins:make /root/catkin_ws/logs/flatland_plugins/build.make.000.log /root/catkin_ws/src/flatland/flatland_plugins/src/world_modifier.cpp:93:59: error: no ‘void flatland_plugins::WorldModifier::CalculateNewWall(double, b2Vec2, b2Vec2, b2EdgeShape&)’ member function declared in class ‘flatland_plugins::WorldModifier’ b2EdgeShape &new_wall) { ^ /root/catkin_ws/src/flatland/flatland_plugins/src/world_modifier.cpp: In member function ‘void flatland_plugins::WorldModifier::AddFullWall(b2EdgeShape*)’: /root/catkin_ws/src/flatland/flatland_plugins/src/world_modifier.cpp:185:49: error: ‘CalculateNewWall’ was not declared in this scope CalculateNewWall(d, vertex1, vertex2, new_wall);