EmulatorPkg: 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: Ray Ni <ray.ni@intel.com>

EmulatorPkg/Library/DxeTimerLib/DxeTimerLib.c

@@ -155,7 +155,7 @@ GetPerformanceCounter (
 UINT64
 EFIAPI
 GetPerformanceCounterProperties (
-  OUT      UINT64                    *StartValue,  OPTIONAL
+  OUT      UINT64                    *StartValue   OPTIONAL,
   OUT      UINT64                    *EndValue     OPTIONAL
   )
 {