DOC: Fix math.

Small typo in the math. I'm not going to merge this yet, because I'm not sure about the new IPython signature and whether it will change when each user makes a commit. Do others see a signature update when they just open and save the notebook?

http://ipython.org/ipython-doc/dev/notebook/security.html#the-details-of-trust

Note also that IPython doesn't support Python 2.6 anymore. I don't know if that means these notebooks can't be run on 2.6 anymore either.