Replace NSPropertyList with NSObject in FBSDKApplicationDelegate.openURL

[CoderBaron]

I've seen on gitter that this was an issue. We changed it in UIApplicationDelegate, so it makes sense to change it here too.
The type in Obj-C is id so in the end it should be NSObject, not NSPropertyList to be correct.

- (BOOL)application:(UIApplication *)application
            openURL:(NSURL *)url
  sourceApplication:(NSString *)sourceApplication
         annotation:(id)annotation;