provider/aws: Specify that aws_network_acl_rule requires a cidr block (#13013)
provider/aws: Specify that aws_network_acl_rule requires a cidr block (#13013)

Fixes: #13011

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSNetworkAclRule_'           2 ↵ ✚
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/23 17:45:25 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSNetworkAclRule_ -timeout 120m
=== RUN   TestAccAWSNetworkAclRule_basic
--- PASS: TestAccAWSNetworkAclRule_basic (41.10s)
=== RUN   TestAccAWSNetworkAclRule_missingParam
--- PASS: TestAccAWSNetworkAclRule_missingParam (21.21s)
=== RUN   TestAccAWSNetworkAclRule_ipv6
--- PASS: TestAccAWSNetworkAclRule_ipv6 (53.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	115.333s
```
kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws