Fix the docsIsoFnBindExample

OK so for me the docsIsoFnBindExample in the directives guide does nothing. That's also the point where I got lost a bit in the documentation. I think it's because transclusion and isolated scopes need to be explained a bit better still. Anyway, the non-working example didn't help my attempts at understanding.

I've fixed the example so it works, but I have little sense if this fix is in line with what you're trying to say in this section.

Here's the plnkr I used to test: http://plnkr.co/edit/EQFTn0?p=preview

@btford, your input is appreciated.