doc: RefsUpdated → RefsFetched — radicle-ci-broker

Since that’s what’s actually looked at (and RefsUpdated isn’t an existing type anyway).

 @startuml
 node -> broker : RefsUpdated event
 node -> broker : RefsFetched event
 broker -> adapter : spawn
 broker -> adapter : send request
 adapter-> engine : trigger run

 @startuml
 node -> broker : RefsUpdated event
 node -> broker : RefsFetched event
 broker -> adapter : spawn
 broker -> adapter : send request
 adapter -> broker : response: run id