Fixed the predicate on NSMetadataQuery for newer OSes

NSMetadataUbiquitousItemDownloadingStatusDownloaded is not an attribute
on NSMetadataItem, it is rather one of the three possible string
constant values of the NSMetadataUbiquitousItemDownloadingStatusKey,
and can therefore not be compared like the now deprecated
NSMetadataUbiquitousItemIsDownloadedKey attribute which used to return
an NSNumber object containing a Boolean.

Thoughts?