README.md does not up to date for version 6.0.1

[PH9]

• AppsFlyerTracker -> AppsFlyerLib
• AppsFlyerTracker.shared().trackAppLaunch() -> AppsFlyerTracker.shared().start()

[PH9]

Oh I just found that it already defined at https://support.appsflyer.com/hc/en-us/articles/115001224823-AppsFlyer-iOS-SDK-Release-Notes