Wrong transmission conditions in Tutorial 11

@oriolcg (I think you did this test)

I was looking at [Tutorial 11](https://gridap.github.io/Tutorials/dev/pages/t011_fsi_tutorial/#Tutorial-11:-Fluid-Structure-Interaction-1).

You consider a steady FSI problem where `us` is the solid velocity. I think it does not make sense, because the solid velocity should be zero.

I think that we want to solve the solid problem using the solid displacement `ds`. In fact, the constitute model you are using makes sense for the displacement, not the velocity.

Then, we want to impose `uf = 0` on the interface.

That would solve the problem, but then the problem is only coupled in one direction. The matrix is upper triangular, since $A_u^{f,s}$ is zero now. It is ok, I would just clarify in the text.

I wanted to agree on this, before starting to make changes.

Commencez par Tutorial 11 et examinez la formulation stationnaire de l’interaction fluide-structure décrite dans l’issue, en vous concentrant sur le déplacement ou la vitesse du solide et sur la condition d’interface. Confirmez les conditions de transmission prévues et le sens du couplage à partir du contexte existant du tutoriel. Le travail est considéré comme terminé lorsque la formulation et le texte explicatif du tutoriel décrivent de manière cohérente le modèle convenu.

Rédigé par le modèle d'indexation à partir du texte de l'issue.