Fix Greengrass subscription docs

[jmettes]

I updated your branch to include changes requested in the AWS Terraform provider PR:
hashicorp#10876

make testacc TEST=./aws TESTARGS='-run=TestAccAWSGreengrassSubscriptionDefinition_'                                                            ==> Checking that code complies with gofmt requirements...                                                       
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAWSGreengrassSubscriptionDefinition_ -timeout 120m
=== RUN   TestAccAWSGreengrassSubscriptionDefinition_basic
=== PAUSE TestAccAWSGreengrassSubscriptionDefinition_basic
=== RUN   TestAccAWSGreengrassSubscriptionDefinition_DefinitionVersion
=== PAUSE TestAccAWSGreengrassSubscriptionDefinition_DefinitionVersion
=== CONT  TestAccAWSGreengrassSubscriptionDefinition_basic
=== CONT  TestAccAWSGreengrassSubscriptionDefinition_DefinitionVersion
--- PASS: TestAccAWSGreengrassSubscriptionDefinition_basic (45.24s)
--- PASS: TestAccAWSGreengrassSubscriptionDefinition_DefinitionVersion (51.61s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	51.641s