Fix documentation on how to run encore

[martinhauke]

Apparently encore needs different parameters than shown in the docs to run. The proposed change seems to work just fine.

Running yarn encore watch results in an error:

✗ yarn encore watch
yarn run v1.22.10
$ /Users/martinhauke/workspace/example/symfony/basic/app/node_modules/.bin/encore watch
Invalid command "watch"

usage encore [dev|prod|production|dev-server]

encore is a thin executable around the webpack or webpack-dev-server executables

Commands:
    dev        : runs webpack for development
       - Supports any webpack options (e.g. --watch)

    dev-server : runs webpack-dev-server
       - --host The hostname/ip address the webpack-dev-server will bind to
       - --port The port the webpack-dev-server will bind to
       - --keep-public-path Do not change the public path (it is usually prefixed by the dev server URL)
       - Supports any webpack-dev-server options

    production : runs webpack for production
       - Supports any webpack options (e.g. --watch)

    encore dev --watch
    encore dev-server
    encore production

error Command failed with exit code 1.

Running yarn encore dev --watch seems to work.

[tgalopin]

Thanks @martinhauke.