schema: fix typo in README
schema: fix typo in README

Fixes: 69e99f504bcf ("schema: describe howto process XML definitions")
Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server