Fix relative path on middleware doc

[nogtk]

Description

I found the link to each included middleware from middleware/index.md was broken, it's defined as a relative path so we don't need middleware directory.

faraday/docs/middleware/index.md

Lines 162 to 168 in 93ef9e0

	The following pages provide detailed configuration for the middleware that ships with Faraday:
	* [Authentication](middleware/included/authentication.md)
	* [URL Encoding](middleware/included/url-encoding.md)
	* [JSON Encoding/Decoding](middleware/included/json.md)
	* [Instrumentation](middleware/included/instrumentation.md)
	* [Logging](middleware/included/logging.md)
	* [Raising Errors](middleware/included/raising-errors.md)

Todos

nothing special

Additional Notes

nothing special

[olleolleolle]

@nogtk I was looking at the rendered website, and those links do refer to the right pages, they seem to do the right thing.

Are you referring to "how the Markdown pages work when rendered on GitHub" or "how to website works when it is rendered"?

Does the change in this PR keep the behaviour of the rendered website?

Here is the section on available middleware and I was able to follow the links.
https://lostisland.github.io/faraday/#/middleware/index?id=available-middleware