Added missing quotation mark in README.md example (#63)

Author: devzx

README.md

@@ -52,7 +52,7 @@ module "alb" {
   security_groups               = ["sg-edcd9784", "sg-edcd9785"]
   log_bucket_name               = "logs-us-east-2-123456789012"
   log_location_prefix           = "my-alb-logs"
-  subnets                       = ["subnet-abcde012", subnet-bcde012a"]
+  subnets                       = ["subnet-abcde012", "subnet-bcde012a"]
   tags                          = "${map("Environment", "test")}"
   vpc_id                        = "vpc-abcde012"
   https_listeners               = "${list(map("certificate_arn", "arn:aws:iam::123456789012:server-certificate/test_cert-123456789012", "port", 443))}"