userdb_ldap_iterate_fields example should use userdb_ldap_iterate_fields, not iterate_attrs

[martinbfrey]

In https://doc.dovecot.org/2.4.1/core/summaries/settings.html#userdb_ldap_iterate_fields the example uses iterate_attrs. This should be userdb_ldap_iterate_fields.

Current:

userdb ldap {
  iterate_filter = (objectClass=smiMessageRecipient)
  iterate_attrs {
    user = %{ldap:mailRoutingAddress}
  }
}

Fixed:

userdb ldap {
  iterate_filter = (objectClass=smiMessageRecipient)
  iterate_fields {
    user = %{ldap:mailRoutingAddress}
  }
}

[cmouse]

this could have been a pull request.

[sirainen]

Also need to remember to fix it in two places there.