docs: fix reference to count of system packages (#24821)
docs: fix reference to count of system packages (#24821)

015e29efe105ddd039e8b395e12cf78a3787ebb3 that introduced this section to the
documentation said “two” here instead of the actual count, three.
9f54cea5c500bb19d2301149460c741e96be1f8b then added a fourth, BLAS/LAPACK.
Rather than trying to keep this leading count in sync, this change just replaces
the wording with something more generic/stable.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack