NetworkPkg: 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: Maciej Rabeda <maciej.rabeda@linux.intel.com>

NetworkPkg/HttpBootDxe/HttpBootImpl.c

@@ -296,7 +296,7 @@ EFI_STATUS
 HttpBootLoadFile (
   IN     HTTP_BOOT_PRIVATE_DATA       *Private,
   IN OUT UINTN                        *BufferSize,
-  IN     VOID                         *Buffer,       OPTIONAL
+  IN     VOID                         *Buffer        OPTIONAL,
      OUT HTTP_BOOT_IMAGE_TYPE         *ImageType
   )
 {