MAINTAINERS: correct the path of UEFI docs

Change the path from doc/uefi to doc/develop/uefi.

Fixes: commit d1ceeeff ("doc: Move UEFI under develop/")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>