message: fix typo in README
  `run -p ... web` sends the argument to docker,
  `run web -p ...` sends the argument to doc.rs
diff: @@ -75,7 +75,7 @@ Docker.
  ```sh
  # This command will start web interface of docs.rs and you can access it from
  # http://localhost:3000/`
  -docker-compose run web -p 3000:3000 start-web-server
  +docker-compose run -p 3000:3000 web start-web-server
  ```
  #### `build` subcommand
  