doc: fix incorrect counter for MPI options (#432636)

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doc
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doc/using/overlays.chapter.md
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### Switching the MPI implementation {#sec-overlays-alternatives-mpi}
-
All programs that are built with [MPI](https://en.wikipedia.org/wiki/Message_Passing_Interface) support use the generic attribute `mpi` as an input. At the moment Nixpkgs natively provides two different MPI implementations:
- [Open MPI](https://www.open-mpi.org/) (default), attribute name
`openmpi`
···
### Switching the MPI implementation {#sec-overlays-alternatives-mpi}
+
All programs that are built with [MPI](https://en.wikipedia.org/wiki/Message_Passing_Interface) support use the generic attribute `mpi` as an input. At the moment Nixpkgs natively provides the following MPI implementations:
- [Open MPI](https://www.open-mpi.org/) (default), attribute name
`openmpi`