Fix an erratum; s/two/three/

There are three options because the new one was added at 6f863cf.

Signed-off-by: Yi EungJun <eungjun.yi@navercorp.com>

docs/userguide/networking/dockernetworks.md

@@ -421,7 +421,7 @@ Once you have several machines provisioned, you can use Docker Swarm to quickly
 form them into a swarm which includes a discovery service as well.
 
 To create an overlay network, you configure options on  the `daemon` on each
-Docker Engine for use with `overlay` network. There are two options to set:
+Docker Engine for use with `overlay` network. There are three options to set:
 
 <table>
     <thead>