Replaced level with layer for OSI model references

- ty to @moxiegirl for the info workflow for patching images.

-link to the v6 svg (grey L2 switch in the top /level/layer/)
http://docs.master.dockerproject.com/article-img/ipv6_switched_network_example.svg

Signed-off-by: Brent Salisbury <brent.salisbury@docker.com>

docs/sources/articles/networking.md

@@ -643,7 +643,7 @@ adapted to the individual environment.
 
 #### Routed Network Environment
 
-In a routed network environment you replace the level 2 switch with a level 3
+In a routed network environment you replace the layer 2 switch with a layer 3
 router. Now the hosts just have to know their default gateway (the router) and
 the route to their own containers (managed by Docker). The router holds all
 routing information about the Docker subnets. When you add or remove a host to