docs: fix config tutorial (#11715)
docs: fix config tutorial (#11715)

This fixes a few typos, adds a hint to `spack config blame` and brings
the variant specification in line with other docs sections.
LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack