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/SnpDxe/Statistics.c

@@ -62,7 +62,7 @@ EFIAPI
 SnpUndi32Statistics (
   IN EFI_SIMPLE_NETWORK_PROTOCOL *This,
   IN BOOLEAN                     Reset,
-  IN OUT UINTN                   *StatisticsSize, OPTIONAL
+  IN OUT UINTN                   *StatisticsSize  OPTIONAL,
   IN OUT EFI_NETWORK_STATISTICS  *StatisticsTable OPTIONAL
   )
 {