Remove static port mapping from README example

This is generally a bad practice when using navy, as it prevents
the same service running in multiple navies - so lets not
advocate it in our README!

Author: tanuck

README.md

@@ -16,7 +16,7 @@ services:
   web:
     image: mycompany/myapp
     ports:
-      - "80:80"
+      - "80"
     depends_on:
       - redis