handbook: make it clear that RFCs should have a single editor
Created by: pdubroy
Per discussion on Slack: https://sourcegraph.slack.com/archives/C02FSM7DU/p1614856694370100
Also fixed up some formatting issues:
Created by: pdubroy
Per discussion on Slack: https://sourcegraph.slack.com/archives/C02FSM7DU/p1614856694370100
Also fixed up some formatting issues: