MdeModulePkg DxeCore: Fix typo in function name
    
    An extra 's' slipped into the FvIsBeingProcessed function
    name. Drop it to fix the typo.
    
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>
    Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>