fix hover style on zoom buttons in examples

`color` & `text-decoration` is actually set by bootstrap, let’s fix it.

Before:
![capture decran 2013-12-18 a 14 29 13](https://f.cloud.github.com/assets/21686/1773487/b24bc50c-67e8-11e3-9e61-c1e285d782eb.png)

After:
![capture decran 2013-12-18 a 14 32 39](https://f.cloud.github.com/assets/21686/1773494/097517de-67e9-11e3-997c-1c25a7c9da05.png)