* Remove `-a/--app` from Quickstart documentation
As mentioned in #2009, simplifying the CLI saw the removal of the `-a/--app` flag. Therefore, the only way to specify the module to import is by setting `FLASK_APP`.
* Remove misleading `either` from CLI help
The CLI help details how to run the application, but still uses the phrasing "either through the `FLASK_APP`...". This likely is an artifact from when `-a/--app` was still present in the CLI.