Mixed Poisson: inconsistency in the description of function spaces Q&V

Thank you for collecting such a rich tutorial about dolfinx etcie.

While reading the section related to the [mixed Poisson](https://jsdokken.com/dolfinx-tutorial/chapter4/mixed_poisson.html#mixed-poisson-with-a-schur-complement-pre-conditioner) problem, the following seems inconsistent

By reversing the description, it becomes consistent with the implementation:
- $p \in Q$ is defined in the 1st order BDM space
- $u \in V$ as the 0th degree DG space