fix(background-mode): update types definitions (#2997)

On the last PR #2982
there was a little typing problem for the .on event listener call.

src/@ionic-native/plugins/background-mode/index.ts

@@ -173,16 +173,14 @@ export class BackgroundMode extends IonicNativePlugin {
  * Register callback for given event.
  * > Available events are `enable`, `disable`, `activate`, `deactivate` and `failure`.
  * @param event {string} Event name
- * @param callback {function} The function to be exec as callback.
- * @param scope {object} The callback function's scope.
  * @returns {Observable<any>}
  */
  @Cordova({
  observable: true,
  clearFunction: 'un',
  clearWithArgs: true
  })
- on(event: string, callback: (...args: any[]) => void, scope?: object): Observable<any> {
+ on(event: string): Observable<any> {
  return;
  }