Replace \texttt{..} with \verb|..|

hrzhu points out that MathJax can't parse \texttt{}. I've replaced it with \verb, which MathJax seems able to handle. Tested with `\verb|forward_difference|(f)(x) = f(x+1)-f(x)` here: http://cdn.mathjax.org/mathjax/latest/test/sample-dynamic.html