Fix typo in index.html for docs

As shown in screenshot below, there is a typo in the documentation for belongsToMany. The only change I've made to the index.html file is to fix this typo from "ore" to "or".

![typo in belongstomany](https://f.cloud.github.com/assets/2865947/2521789/22d47520-b4b1-11e3-8d66-0713911f92f6.png)