Update helmet.md

Old import was not working anymore, error:

> Argument of type 'typeof import("/.../node_modules/fastify-helmet/index")' is not assignable to parameter of type 'FastifyPluginCallback<any, Server> | FastifyPluginAsync<any, Server> | Promise<{ default: FastifyPluginCallback<any, Server>; }> | Promise<...>'.

fastify-helmet: 5.3.0
helmet: 4.4.1
@nestjs/platform-fastify: 7.6.14 & 8.0.0-alpha.3

Author: Mick Lawitzke

content/security/helmet.md

@@ -33,7 +33,7 @@ $ npm i --save fastify-helmet
 [fastify-helmet](https://github.com/fastify/fastify-helmet) should not be used as a middleware, but as a [Fastify plugin](https://www.fastify.io/docs/latest/Plugins/), i.e., by using `app.register()`:
 
 ```typescript
-import * as helmet from 'fastify-helmet';
+import { fastifyHelmet } from 'fastify-helmet';
 // somewhere in your initialization file
 app.register(helmet);
 ```