Update acl comment to be more clear.

playbooks/edx-east/edx_vpc.yml

@@ -21,9 +21,9 @@
         wait: yes
       register: created_vpc
 
-    # A public network acl is created
-    # when a vpc is created so each VPC
-    # should have one.
+    # A default network acl is created when a vpc is created so each VPC
+    # should have one but we create one here that allows access to the
+    # outside world using the internet gateway.
     - name: create public network acl
       ec2_acl:
         profile: "{{ profile }}"