UefiCpuPkg/MpInitLib: Change AP Index variable name.

UefiCpuPkg/MpInitLib: Change AP Index variable name.

Original AP index variable name not well express the meaning
of the variable. Also this name is better used in later patch.
So change the variable name for better understanding.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jeff Fan <vanjeff_919@hotmail.com>