Point from HTTP-Client docs to DI

This issue was originally opened in https://github.com/aurelia/site-generator/issues/30. I've updated the reference here.

Hey there!

I'm currently porting a small demo over to Aurelia from Angular 9 and love it so far!
I'm not sure, whether this repo is still used for updating the website, but …

**What is the motivation / use case for changing the behavior?**
In [HTTP Services](https://github.com/aurelia/documentation/blob/e7b2e8bd429020b90121db76ec328d512cc3d89d/current/en-us/7.%20plugins/1.%20http-services.md#bring-your-own-polyfill) you are currently describing, how to build AJAX requests.

You also recommend (typo in the docs!) to:

> Instead, we reccommend that you create one or more service classes that encapsulate all HTTP access behind a friendly, application-specific API.

I'd suggest to place a link there to [Dependency Injection](https://github.com/aurelia/documentation/blob/master/current/en-us/3.%20fundamentals/3.%20dependency-injection.md#injection), because that would describe how the service class would be written.

I discovered the page via a DDG-Search finally.

Thanks for making Aurelia :-)
The project evolved way better (watching your progress since the Skeleton times ;-) )

Open current/en-us/7. plugins/1. http-services.md and locate the “Bring your own polyfill” section and its recommendation about service classes. Add a link there to the Dependency Injection documentation and correct the “reccommend” typo. Done means the link resolves and the updated wording appears in the HTTP Services documentation.

Written by the indexing model from the issue text.