MdeModulePkg CapsuleApp: Index need be decimal for -P GET option

MdeModulePkg CapsuleApp: Index need be decimal for -P GET option

Also adjust the help information to be not too long to be suitable
for different display resolutions.

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>