Fix the result message in test_network.md

Fix the result message on running "./network.sh createChannel"

The result message format has been changed from the change of
test-network/scripts/createChannel.sh in hyperledger/fabric-samples
since
b690d8f Stop using deprecated outputAnchorPeersUpdate in test-network (#394)

Signed-off-by: Justin Yang <justin.yang@themedium.io>

docs/source/test_network.md

@@ -191,7 +191,7 @@ channel with the default name of `mychannel`:
 If the command was successful, you can see the following message printed in your
 logs:
 ```
-========= Channel successfully joined ===========
+Channel 'mychannel' joined
 ```
 
 You can also use the channel flag to create a channel with custom name. As an