Fixed usage of wrong string

Author: litetex

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -171,7 +171,7 @@ public static void playOnBackgroundPlayer(final Context context,
         Toast.makeText(
                 context,
                 PlayerHolder.getInstance().getType() == PlayerType.AUDIO
-                    ? R.string.background_player_playing_toast
+                    ? R.string.background_player_already_playing_toast
                     : R.string.background_player_playing_toast,
                 Toast.LENGTH_SHORT)
                 .show();