README: fix instruction for network test

I was setting up an OSv with network today, following README instructions
and noticed that to run the test an invoke command is missing.

Maybe it's a syntax change from older versions.

Signed-off-by: Eduardo Piva <efpiva@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

README.md

@@ -99,7 +99,7 @@ route add default gw 192.168.122.1
 Test networking:
 
 ```
-test TCPExternalCommunication
+test invoke TCPExternalCommunication
 ```
 
 Running Java or C applications that already reside within the image: