SecurityPkg: Fix spelling mistake for occurred

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2361

Cc: Qi Zhang <qi1.zhang@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c

@@ -1465,7 +1465,7 @@ IsAllowedByDb (
 
   //
   // Fetch 'dbx' content. If 'dbx' doesn't exist, continue to check 'db'.
-  // If any other errors occured, no need to check 'db' but just return
+  // If any other errors occurred, no need to check 'db' but just return
   // not-allowed-by-db (FALSE) to avoid bypass.
   //
   DbxDataSize = 0;