config: add example on external_address (#6621)

## Description

It confused many people what they were supposed to add here. For chains with large genesis files you will only see the error after Initgenesis. Best to add a small sentence to provide better UX

config/toml.go

@@ -280,7 +280,8 @@ laddr = "{{ .P2P.ListenAddress }}"
 # Address to advertise to peers for them to dial
 # If empty, will use the same port as the laddr,
 # and will introspect on the listener or use UPnP
-# to figure out the address.
+# to figure out the address. ip and port are required
+# example: 159.89.10.97:26656
 external-address = "{{ .P2P.ExternalAddress }}"
 
 # Comma separated list of seed nodes to connect to