command: use correct port mapping syntax in examples

command/assets/example-short.nomad

@@ -3,7 +3,9 @@ job "example" {
 
   group "cache" {
     network {
-      port "db" {}
+      port "db" {
+        to = 6379
+      }
     }
 
     task "redis" {

command/assets/example.nomad

@@ -151,7 +151,9 @@ job "example" {
     #     https://www.nomadproject.io/docs/job-specification/network
     #
     network {
-      port "db" {}
+      port "db" {
+        to = 6379
+      }
     }
 
     # The "service" stanza instructs Nomad to register this task as a service