firmware: Fix grammar in sysfs-firmware-dmi doc

Fix the grammar in describing the position attribute of DMI entries in
the dmi-sysfs module.  While here, make a couple other small clarifying
fixups to the docs.

Reported-by: <Valdis.Kletnieks@vt.edu>
Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>