fix typos/grammos in Documentation/edac.txt

Trivial patch fixes some typos in the Documentation/edac.txt file.

Acked-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>