fix pi tutorial for mpi on pi cluster

[laszewsk]

looks like https://github.com/cloudmesh/pi/blob/main/content/en/tutorial/mpi/index.md
is missing the installation of the MPI command? I made a comment in the tutorial
there is some other place where some documentation is incomplete

[laszewsk]

Maybe we need just one tutorial. We start with README.md and when done we move this to piplanet
and just point to it from piplanet?