fix(JitsiConference.js): Adds punctuation.

JitsiConference.js

  * Checks if the participant given by participantId is currently included in
  * the last N.
  * @param {string} participantId the identifier of the participant we would
- * like to check
+ * like to check.
  * @return {boolean} true if the participant with id is in the last N set or
- * if there's no last N set, false otherwise
+ * if there's no last N set, false otherwise.
  */
 JitsiConference.prototype.isInLastN = function(participantId) {
     return this.rtc.isInLastN(participantId);