edac: adjust docs location at MAINTAINERS and 00-INDEX

Update MAINTAINERS to reflect the location of edac.rst and ras.rst.

In the case of 00-INDEX, there's already an entry to the admin-guide,
so all we need to do is to remove the entry there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>