Documentation fix: build_system configure_args for #21760 (#21761)
Documentation fix: build_system configure_args for #21760 (#21761)

Corrects the signature for configure_args (and therefore configure_vars)
in documentation on RPackage build system to match the code
See issue #21760
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack