MAINTAINERS: use the correct efi-stub location

Commit 4171fe2f8a47 ("EFI stub documentation updates") moved the file,
update the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>