monopam: trim opam-repository speculation from comments
The lint and root.ml comments claimed dev-repo: ends up in
opam-repository to direct users at the canonical home. monopam
push routes via sources.toml, not via the .opam dev-repo field,
so that justification is more aspirational than concrete.
Keep the rule (dune-project (source ...) must match
sources.toml's upstream when set) and drop the over-claimed
rationale.