Fix federation_reader listeners doc as per PR review
Fix federation_reader listeners doc as per PR review

Signed-off-by: Jason Robinson <73c4b53c33b2f03b4e26d9578bea93e0ed8e16b0@matrix.org>

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse