Updated README.md Swift example
Updated README.md Swift example

Change Swift example code to use the `CocoaLumberjackSwift` module instead of `CocoaLumberjack` so that the convenience functions `DDLogWarn`, `DDLogError` etc work.
jum/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,sushichop/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,jum/CocoaLumberjack,DD-P/CocoaLumberjack,sushichop/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,sushichop/CocoaLumberjack,DD-P/CocoaLumberjack,jum/CocoaLumberjack,DD-P/CocoaLumberjack,sushichop/CocoaLumberjack,jum/CocoaLumberjack