Updated cookies.md with register signature

at 9.0.4 we should return a callback, or we've got a ts error:

TS2345: Argument of type 'typeof fastifyCookie' is not assignable to parameter of type 'FastifyPluginCallback  | FastifyPluginAsync  | Promise{ default: FastifyPluginCallback ; }> | Promise...>'.

content/techniques/cookies.md

@@ -71,7 +71,7 @@ const app = await NestFactory.create<NestFastifyApplication>(
   AppModule,
   new FastifyAdapter(),
 );
-await app.register(fastifyCookie, {
+await app.register(() => fastifyCookie, {
   secret: 'my-secret', // for cookies signature
 });
 ```