Maintainers.txt: move StandaloneMmPkg to the right spot

Maintainers.txt: move StandaloneMmPkg to the right spot

Place StandaloneMmPkg between SourceLevelDebugPkg and UefiCpuPkg, where it
belongs in lexicographical order. (Right now it succeeds
UnitTestFrameworkPkg, which is a disorder.)

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2778
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200603160627.3594-4-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>