MdePkg: fix comments typo about EFIAPI for X64

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Gao, Liming <liming.gao@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16299 6f19259b-4bc3-4df7-8a09-765794883524