The Cylinder 5 misfiring which would be engine code P0305 when tested with a OBD Tool just means that the cylinder is not igniting the air and fuel mixture correctly and could be caused by a number of different things.
The cylinder 5 misfire with engine code P0305 could be caused by a bad spark plug, bad ignition coil, low fuel pressure, dirty fuel injectors etc.
If it's the spark plug then all spark plugs should be changed at once to give the engine the best performance.
Engine misfires can become a serious problem if left unfixed and could potentially damage your engine so get it fixed as soon as possible.