Moves Tree Contents so that simpsons data tree in example is defined.

[flamingbear]

This is a clone of #300 because I nuked my fork without thinking.

Since I'm reading the docs and don't know sphinx very well, I saw a problem with the is_hollow description and fixed it.

It may make more sense to have Tree Contents: Hollow Trees above Manipulating Trees: Subsetting Tree Nodes, but the simpsons DataTree is undefined. Rather than create a new tree for this example I just moved it.

In addition to moving it I changed the description from a python method to the py:class so that the rendering didn't include the incorrect parens.

The updated docs after rebuilding show the tree is not hollow rather than NameError: name 'simpsons' is not defined as it did before

New image generated:

Also increased clarity of the dt.match() operation by printing the result.

And fixed a typo. wo => twoj

[TomNicholas]

Thanks @flamingbear. FYI the reason I added the content about "hollow trees" was in preparation for #271.