Fixed property key attribution in generic createSimpleSchema<T>

[jglanz]

While using createSimpleSchema<T> I had noticed that key/property validation wasn't working and had a peek at the typings.

I noticed that while T was accepted as a generic, it was not used with regard to the property mappings provided, so a 3 character change was made :)

[mweststrate]

Thanks for catching!

[coveralls]

coverage: 90.239%. remained the same
when pulling c0c1011 on jglanz:master
into f8b9196 on mobxjs:master.