fixing pattern for preg match all

The pattern was mis-identifying chapter titles with the string 'math' as containing mathml markup. This new pattern is more specific, less greedy.

![screen shot 2014-02-17 at 10 22 22 am](https://f.cloud.github.com/assets/2048170/2188112/8d228886-9800-11e3-9e86-36ce22512c14.png)

The following chapter title triggered the error: 

![screen shot 2014-02-17 at 10 25 42 am](https://f.cloud.github.com/assets/2048170/2188139/f530721c-9800-11e3-8128-2f213b9b9142.png)