Correct Type in Comment On getPresentationPeriod_

"is" was used where we likely meant "if".

Change-Id: I75f38a4b9eb498101911a7e98ff51d86fc83597f

lib/player.js

@@ -3567,7 +3567,7 @@ shaka.Player.prototype.getSelectableText_ = function() {
 };
 
 /**
- * Get the period that is on the screen. This will return |null| is nothing
+ * Get the period that is on the screen. This will return |null| if nothing
  * is loaded.
  *
  * @return {?shaka.extern.Period}