Update documentation using basic auth middleware

[peterberkenbosch]

Documentation on using basic auth is incorrect. The deprecation warning shows the right information though. This PR updates the documentation so it matches the deprecation warning and works as expected.

When using the current documented basic auth middleware it will render the following error:

ArgumentError: wrong number of arguments (given 4, expected 3)

[iMacTia]

@peterberkenbosch thanks for trying to fix this and apologies about the confusion 🙏
As @olleolleolle explained in his comment, the documentation website is currently built based on the main branch, which already incorporates the v2.0 changes.
That's why the example is incorrect for people using Faraday 1.x.

This is not the first report and we're exploring how to effectively tackle this issue as our current tools don't allow us to have version switching in the docs.

I'll be closing this but will try to put something temporary in place to avoid further confusion