docs: fastify plugin registration should be awaited

Fastify plugin registration happens asynchronously and should be awaited.

Closes #2265

Author: Christian Potdevin

content/techniques/cookies.md

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

content/techniques/sessions.md

@@ -77,7 +77,7 @@ const app = await NestFactory.create<NestFastifyApplication>(
   AppModule,
   new FastifyAdapter(),
 );
-app.register(secureSession, {
+await app.register(secureSession, {
   secret: 'averylogphrasebiggerthanthirtytwochars',
   salt: 'mq9hDxBVDbspDR6n',
 });