Fixed complicated grammar

On the home page, in the section under the header titled "Native focused", there is an occurrence of complicated grammar, which may confuse some readers. 

I suggest correcting <b>off of</b> to <b>on</b> in the following sentence (also see the image below). 

<b>Original Sentence</b>
 "Ionic is modelled <b>off of</b> popular native mobile development SDKs....."

<b>Correction</b>
 "Ionic is modelled <b>on</b> popular native mobile development SDKs....."

![screen shot 2014-05-29 at 12 18 40](https://cloud.githubusercontent.com/assets/4481885/3122374/75dbe5fe-e766-11e3-98d1-718dfbdd7075.png)
Merge pull request #180 from omkarv/patch-2

docs(index): s/modeled off of/modeled on