You can use whichever you want, 15 or 22pF. In fact, the industrial grade boards mounts 15pF capacitors as stated in the components list. I've reviewed my schematics and the used capacitors are the 15pF ones. Maybe there is in the website an obsolete version of the schematic file? Will review the main post and check that.
This is not like an audio equipment, where the quality and the value of the capacitors are very important to the sound quality. Here... it works or it doesn't works. So, it will work well with any of the both values.