Updated docs with correct type for string dates

[hugofelp]

It is confusing to have type Date in the documentation, when, in reality, a String is expected.

[LorienHW]

A little context relating to this issue - We are having to update our code now that calls RNCalendarEvents API's because we were previously passing Date objects to the API's, and it used to work fine. We now have upgraded to RN 59, and now we are getting errors in iOS. I think something must have changed in the RN bridge related code that now breaks if you pass JS Date objects across the bridge to be converted to NSDate objects. Updating our code to pass strings instead works correctly.