BaseTools: Correct the variable name
    
    the commit bff74750 introduce a undefined variable name 'scope' cause build
    failure, it should use 'Scope'.
    
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>