provider/aws: Specify that aws_network_acl_rule requires a cidr block

[stack72]

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

[grubernaut]

LGTM!!

[ghost]

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