Update `Pagy:: Backend` comment

[leonvogt]

Hi 👋

I've noticed that the overriding of pagy_get_vars stopped working after the 9.0.0 upgrade.
It seems this method was removed during this refactoring.

In my use case, I was able to switch to overriding pagy_get_count instead.

I've updated the comment to reflect this change.

[ddnexus]

This PR requires a rebase on master in order to pass the tests.

[ddnexus]

Thank you!