Update README.md to account for misnamed method

In v0.5.4 onwards the method is no longer `execute` but looks to be `read`

Please see [here](https://github.com/OfficeDev/Office-365-SDK-for-iOS/commit/d7574f9278d2ff15da3bf2a961d7a335de2caadc#diff-70ea766cc5ac2921b686c18348cc5161R95) for the difference.  

![screen shot 2014-11-04 at 3 08 17 pm](https://cloud.githubusercontent.com/assets/839972/4918713/b6838b6a-64f3-11e4-936f-cb305e4496ad.png)

This cause significant confusion for me since even the sample apps use execute and seem to work but I was having build failures when following the readme instructions