Change equation formatting to code formatting

`build_finetune_functions` is being rendered as a broken math equation, not a code snippet.

![1735cdfbfbfc569c028c02ee182eb0c6b1e00475](https://f.cloud.github.com/assets/435548/1823495/1b0422da-717e-11e3-975e-c105faddbef5.png)