Fix reference in known_hosts doc (#2563)

The format is described in sshd(1), not ssh(1).

lib/ansible/modules/extras/system/known_hosts.py

@@ -37,7 +37,7 @@ options:
     default: null
   key:
     description:
-      - The SSH public host key, as a string (required if state=present, optional when state=absent, in which case all keys for the host are removed). The key must be in the right format for ssh (see ssh(1), section "SSH_KNOWN_HOSTS FILE FORMAT")
+      - The SSH public host key, as a string (required if state=present, optional when state=absent, in which case all keys for the host are removed). The key must be in the right format for ssh (see sshd(1), section "SSH_KNOWN_HOSTS FILE FORMAT")
     required: false
     default: null
   path: