docs: (#1200)

Fixes ordering of command-line arguments. `substrate --help` shows usage as substrate `[FLAGS] [OPTIONS] [SUBCOMMAND]`, where `--chain=staging` is one of the possible OPTIONS, and `build-spec` is a possible SUBCOMMAND.
The ordering should be `substrate --chain=staging build-spec` instead.

Relates to issue raised by user attente in Riot https://matrix.to/#/!HzySYSaIhtyWrwiwEV:matrix.org/$154385734133299JuBZU:matrix.parity.io