In step 2 of the Scaling an Existing Cluster procedure within the Atlas documentation, the table states the following:

Replication Factor

You can change the number of members in your cluster’s replica set. The replica set spans a minimum of three availability zones, and each member of the replica set runs on a separate instance. [...]

However, Atlas now also supports deploying clusters into regions that only contain two availability zones; the documentation should be updated to reflect this.