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.