MdeModulePkg CapsuleApp: Refine -N option help information

MdeModulePkg CapsuleApp: Refine -N option help information

-N option is used to append a Capsule Header to an existing
FMP capsule image with its ImageTypeId supported by the system.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>