docs(): fix options for prepareInterstitial (#1535)
docs(): fix options for prepareInterstitial (#1535)

Using the example in the previous version of this document, calling prepareInterstitial(adId) resulted in an
"-[__NSCFString count]: unrecognized selector sent to instance"
exception. This was alleviated by changing the call to prepareInterstitial({adId: adId}) 
(e.g. passing in an options object vs. just the adid string)
driftyco/ionic-native,driftyco/ionic-native,driftyco/ionic-native