EmulatorPkg:Change DEC_VERSION to DEC_SPECIFICATION

edk2 DEC specification document only knows about DEC_SPECIFICATION,
so using DEC_VERSION in [Defines] section in EmulatorPkg.dec is not
correct.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Wenyi Xie <xiewenyi2@huawei.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>