notebook examples title cell

``nbgenerate.py`` requires the title of notebook examples to be in a cell marked as "heading". The two notebooks I moved in my last PR didn't have that and so were doing something strange during docs build. This should fix this problem. 

I built the docs locally using IPython 1.0 (good job on that, BTW, @jseabold) and all looks fine. This is a trivial fix, so I'll merge if I don't see opposition within the next couple days.