Fix command example

[bumbummen99]

https://docs.adonisjs.com/guides/database/migrations

It only says:

You can also create a Lucid model and the migration together by running the `node ace make -m' flag.

[bumbummen99]

Also when running the command it does return the following error:

[ error ]  Expected "--migration" to be a valid "boolean"

When running like so it does work: node ace make:model -m=true MyModel.

Can't this just be a flag?

[thetutlage]

Thanks for the fix