MAINTAINERS: fix up exfat directory location

I messed up on the exfat MAINTAINER entry, the code is in
drivers/staging/exfat/ not fs/exfat/

Cc: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Reported-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>