fix(iframe_api): s/getNumberOfParticipant/getNumberOfParticipants

doc/api/api.md

 
 You can get the number of participants in the conference with the following code:
 ```
-var numberOfParticipants = api.getNumberOfParticipant();
+var numberOfParticipants = api.getNumberOfParticipants();
 ```
 
 You can remove the embedded Jitsi Meet Conference with the following code:

modules/API/external/external_api.js

  * NOTE: the local participant is included.
  * @returns {int} the number of participants in the conference.
  */
-JitsiMeetExternalAPI.prototype.getNumberOfParticipant = function() {
+JitsiMeetExternalAPI.prototype.getNumberOfParticipants = function() {
     return this.numberOfParticipants;
 };