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

[hrzhu](https://github.com/kisonecat/m2o2c2/commit/6a84abff7ac4312315ab4f68d54763ce5d7c11fa#commitcomment-5768878) 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). Produces the following:

![mathjax_rendering](https://f.cloud.github.com/assets/4433943/2493294/0b65f96c-b28c-11e3-8914-2ef8ec65afa1.png)