[material][fluidmatrixinteraction] Fix doc to include Brooks Corey law.

What this MR does / why does DuMux need it:
Within the Brooks Corey law, we refered to the vanGenuchten law. The documentation was probably simply copied without changing the model name.

Before you request a review from someone, make sure to revise the following points:

• do the test pipelines pass? (see guide on [how to run pipelines for a merge request](https://git.iws.uni-stuttgart.de/dumux-repositories