fix the return type documentation

src/system/system.controller.ts

@@ -8,7 +8,7 @@ export class SystemController {
   @Get('/timestamp')
   @ApiResponse({
     description: 'The current ZULU timestamp of the system',
-    type: String,
+    type: Number,
   })
   async timestamp(): Promise<number> {
     return new Date().getTime();