[documentation] add --config argument for -- run-migrations

[dorin100]

# SMASHPGPASSFILE=config/pgpass stack run smash-exe -- run-migrations --mdir ./schema
Missing: --config FILEPATH

Usage: smash-exe run-migrations --config FILEPATH --mdir ARG [--ldir ARG]
  Run the database migrations (which are idempotent).

• it works if you add the --config config/shelley-qa-config.yaml argument