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.

Tutorial 11 から始め、issue に記載されている定常流体構造連成の定式化を確認し、固体の変位または速度と界面条件に焦点を当てます。既存のチュートリアルの文脈に照らして、意図された伝達条件と連成方向を確認します。チュートリアルの定式化と説明文が合意されたモデルを一貫して記述していれば完了です。

索引モデルが issue の本文から書いたものです。