BaseTools/GenSec: Fix spelling mistake

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2345



Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>