SignedCapsulePkg: Change OPTIONAL keyword usage style

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3760

Update all use of ', OPTIONAL' to ' OPTIONAL,' for function params.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

SignedCapsulePkg/Library/EdkiiSystemCapsuleLib/EdkiiSystemCapsuleLib.c

@@ -661,7 +661,7 @@ CapsuleAuthenticateSystemFirmware (
 VOID
 EFIAPI
 EdkiiSystemCapsuleLibPcdCallBack (
-  IN        CONST GUID        *CallBackGuid, OPTIONAL
+  IN        CONST GUID        *CallBackGuid  OPTIONAL,
   IN        UINTN             CallBackToken,
   IN  OUT   VOID              *TokenData,
   IN        UINTN             TokenDataSize