Update middleware.md

[amir-mohammad-HP]

the type definition explanation for adding a new property to context was missed in documentation for modifying the context in dependency injection

i have explained how that would be done

[thetutlage]

The same is explained in the "Extending AdonisJS" guide. https://docs.adonisjs.com/guides/extend-adonisjs So it is not needed in the HttpContext guide.