Improve uiop:collect-sub*directories example

[mirkov]

The example in the section on walking/traversing directories lists the arguments of the uiop:collect-sub*directories

It would be good to reorder the list in the order of this function's arguments like so:

• a directory
• a collectp function
• a recursep function
• a collector function

That will make the example more readable

[pnathan]

I like the idea.