docs: fix compiler config (#12330)
docs: fix compiler config (#12330)

It's no longer possible to set compiler flags under as an entry under
"paths" in compilers.yaml; instead the user must list these under the
"flags" section. This updates the docs accordingly.
LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack