IntelFsp2Pkg/Tools: Fix a typo issue

IntelFsp2Pkg/Tools: Fix a typo issue

Error message:
raise Exception ("'%s' is not a valid directory!" % FvDir)
NameError: name 'FvDir' is not defined

FvDir should be fvDir.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Yunhua Feng <fengyunhua@byosoft.com.cn>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>