Update spack.md

docs/tursa-user-guide/spack.md

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 [Spack](https://github.com/spack/spack) is a package manager, a tool to assist
 with building and installing software as well as determining what dependencies
 are required and installing those. It was originally designed for use on HPC
-clusters, where several variations of a given package may be installed alongside
+systems, where several variations of a given package may be installed alongside
 one another for different use cases -- for example different versions, built
 with different compilers, using MPI or hybrid MPI+OpenMP. Spack is principally
 written in Python but has a component written in Answer Set Programming (ASP)