You can probably run a relay on Tails by modifying the torrc the same way as in the guide. However, if you boot and shut down your Tails often, you shouldn't run a relay on it. Tor relays should be stable, with many days of uptime. It's bad for the Tor network when relays go offline a lot, because it takes time to update the directory and get all clients to know about the currently running relays. The network would become more inefficient if clients were trying to connect to a lot of relays that are offline. Tails is a non-persistent mobile operating system. Relays should be run on stable setups.