fix port '80' is already in use

If the voting app is deployed using production params, the result service
fails to deploy as port 80 already is taken by vote service

Steps to reproduce:
voting-app$ docker-app render
  --parameters-files voting-app.dockerapp/parameters/production.yml
| docker stack deploy --compose-file - voting

Signed-off-by: Andreas Amstutz <andreasamstutz@gmail.com>

Author: Andreas Amstutz

examples/voting-app/README.md

@@ -169,7 +169,7 @@ vote:
 
 # Result.
 result:
-  port: 80
+  port: 81
   replicas: 5
 ```
 

examples/voting-app/voting-app.dockerapp/parameters/production.yml

@@ -5,5 +5,5 @@ vote:
 
 # Result.
 result:
-  port: 80
+  port: 81
   replicas: 5