fix(appium): closeApp supports both Android/iOS (#4046)

Author: kobenguyent

docs/helpers/Appium.md

@@ -871,7 +871,7 @@ Close the given application.
 I.closeApp();
 ```
 
-Returns **[Promise][6]<void>** Appium: support only iOS
+Returns **[Promise][6]<void>** Appium: support both Android and iOS
 
 ### appendField
 

lib/helper/Appium.js

@@ -1424,10 +1424,10 @@ class Appium extends Webdriver {
    *
    * @return {Promise<void>}
    *
-   * Appium: support only iOS
+   * Appium: support both Android and iOS
    */
   async closeApp() {
-    onlyForApps.call(this, 'iOS');
+    onlyForApps.call(this);
     return this.browser.closeApp();
   }