docs: Do not use automatic title in fixture reference

[alfredodeza]

It creates odd wording otherwise. Keep the reference, update the title
using Sphinx notation.

For example, in the cache section it reads:

Here is a basic example plugin which implements a pytest fixtures: explicit, modular, scalable which re-uses previously created state across pytest invocations:

With this change it reads:

Here is a basic example plugin which implements a fixture which re-uses previously created state across pytest invocations:

This PR doesn't address all the references to fixtures (I checked them all!), because in some other situations it does read fine.

[blueyed]

Thanks!