Fix erroneous comment.
Fix erroneous comment.

The comment for `isEmpty()` does not match the way it operates.
tgyhlsb/RxSwiftExt,tgyhlsb/RxSwiftExt,RxSwiftCommunity/RxSwiftExt,RxSwiftCommunity/RxSwift-Ext,RxSwiftCommunity/RxSwift-Ext,RxSwiftCommunity/RxSwiftExt,RxSwiftCommunity/RxSwiftExt