`prefix` parameter is incompletely defined for `process-issue` command

[caufieldjh]

The prefix parameter in the process_issue CLI command is incompletely defined and doesn't get used anywhere.
See

ontobot-change-agent/src/ontobot_change_agent/cli.py

Line 226 in 7c7e421

def process_issue(

This means a command like

ochange process-issue src/ontology/mondo-edit.obo -r caufieldjh/mondo -n 10 -g  -p MONDO

will fail with

Error: Got unexpected extra argument (MONDO)