docs(connector-fabric): fix API call testing tutorial in README.md

[petermetz]

Description

I renovated the Fabric Connector's README.md file recently and got most of the container image related tutorials working except for the last one which is the end to end one that pull up the Fabric2 AIO ledger and then configures an API server and sends some requests to the connector for processing.

It's almost there but I had to leave this in the README for now:

FIXME: This does not work. We need to (at the very least)
1. add the a keychain plugin to the API server config
2. load the keychain up with credentials that can be used to access the Ledger
3. reference the keychain entry created in step 2 as the signing credential below.

Acceptance Criteria

1. The tutorial steps work and are concise, easy to understand and execute on the terminal