Example XML should have `version=2.1`

[jwfraustro]

The XSD schema fixes the VOEvent element's version at 2.1. The version attribute of the example XML should probably be updated.

VOEvent/VOEvent.tex

Lines 1510 to 1514 in 00f6004

	version="2.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:voe="http://www.ivoa.net/xml/VOEvent/v2.0"
	xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0
	http://www.ivoa.net/xml/VOEvent/VOEvent-v2.0.xsd">

[BaptisteCecconi]

This is fixed in #33