Fix custom topology example; outdated import

Reported-by: Julius Bachnick

custom/topo-2sw-2host.py

@@ -10,7 +10,8 @@ Adding the 'topos' dict with a key/value pair to generate our newly defined
 topology enables one to pass in '--topo=mytopo' from the command line.
 """
 
-from mininet.topo import Topo, Node
+from mininet.topo import Topo
+from mininet.node import Node
 
 class MyTopo( Topo ):
     "Simple topology example."