[relax-ng] rngnx and UTF8

[Michael Fitzgerald <mike@wyeast.net>]

I noticed that rngnx accepts UTF8 as a second argument but not UTF-8 and
thus writes:

<?xml version="1.0" encoding="UTF8"?>

I know Java uses 'UTF8' (such as in OutputStreamWriter) but XML processors
don't like it very much.

Mike