fix: example mentions web::Data but refers to plain state struct

[LeoniePhiline]

The mutable app state example at https://ntex.rs/docs/application#shared-mutable-state mentions web::Data.

Since what is being created is a plain struct, the example's comment is reworded to // Note: app state created _outside_ HttpServer::new closure.

Closes #10

[leon3s]

Thanks!