Fix --conf documentation

[Patricol]

Describe the bug Documentation states that --conf specifies "A path to the configuration file (by default, flowerconfig.py)"

To Reproduce Attempting to provide a path always fails. (--conf=relative_path/flowerconfig.py -> The module relative_path/flowerconfig was not found., --conf=/absolute_path/flowerconfig.py -> The module /absolute_path/flowerconfig was not found., etc.)

Expected behavior Documentation should state that it's a python-import-style module. --conf=relative_path.flowerconfig works.

System information Using flower 1.0.0, celery 4.4.7, and python 3.6.9. (unfortunately)

It was very hard to research this bug/behavior; the main information I could find was in the incorrect documentation. Hopefully this fix can be pushed to docs soon to avoid another person wasting hours figuring this out. Thank you!