Cannot use :filter option with fixture-def

[the2bears]

This is pretty minor, only needing a change documentation.

If the :filter option is used when creating a fixture-def, a java.lang.IllegalAccessError will be thrown, due to the fixture being final: https://github.com/libgdx/libgdx/blob/master/extensions/gdx-box2d/gdx-box2d/src/com/badlogic/gdx/physics/box2d/FixtureDef.java

Filters may be changed through filter! or by obtaining a reference to the filter from the fixture-def and setting the fields on the filter directly.