OvmfPkg: 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: Andrew Fish <afish@apple.com>

OvmfPkg/AcpiPlatformDxe/BootScript.c

@@ -190,7 +190,7 @@ STATIC
 VOID
 EFIAPI
 AppendFwCfgBootScript (
-  IN OUT VOID *Context,              OPTIONAL
+  IN OUT VOID *Context               OPTIONAL,
   IN OUT VOID *ExternalScratchBuffer
   )
 {