fixed docs: method calculates with kilometers, not with meters

This method actually calculates with kilometers, not with meters as specified in the docs.
For now i fixed the docs.
Better would be to discuss, if it makes more sense to prepend a division by 1000 inside the method before working with the distance-parameter to make the code behave according to the docs.

I suggest leaving it as is due to the fact that in everyday-use of GIS-stuff, we usually calculate routes for cars and bicycles, stuff like that. In this field, the kilometer is a better handleable dimension.