Fixing a formatting error in the custom display example notebook.

Just a small typo fix.  Right now the underscores are not rendered since they aren't escaped.  This is important because the underscores are part of the method names for these functions - something that isn't obvious when reading the notebook.  See: http://nbviewer.ipython.org/urls/raw.github.com/ipython/ipython/master/examples/notebooks/Custom%20Display%20Logic.ipynb

I wrapped all the method names in backticks to avoid confusion in the future.