Wrong flag in tutorial for file manifest

[jemma-nelson]

The tutorial suggests using -f to load a manifest from the filesystem: https://bulker.databio.org/en/latest/tutorial/ with bulker load demo -f http://big.databio.org/bulker/bulker/demo.yaml

It seems that this should instead instruct the user to use the -m flag, as shown in the usage:

  -m MANIFEST, --manifest MANIFEST
                        File path to manifest. Can be a remote URL or local
                        file.