fix mathjax example notebook so that it works with pandoc

[before](http://nbviewer.ipython.org/github/ipython/ipython/blob/14625dfdff02b9e57588853aba6c43bbd4adf8b2/examples/notebooks/Typesetting%20Math%20Using%20MathJax.ipynb) and [after](http://nbviewer.ipython.org/github/minrk/ipython/blob/mathjax-example/examples/notebooks/Typesetting%20Math%20Using%20MathJax.ipynb).