Update part-three.rst

I ran into a few issues while working my way through the TreeBehavior portion of the tutorial. There wasn't a mention of removing the validators from the Categories model in addition to the lft and rght input fields from the Categories view.ctp and edit.ctp files.

This of course prevented me from saving my model. I either had to make the form fields hidden (i.e., $this->Form->hidden('lft')) but since the documentation already mentions removing them I presume removing the validators from the model was the best practice.

Please let me know if you have any questions, comments, or observations.