Removed SNS subject argument default from readme

Author: olgierdg

README.md

@@ -60,7 +60,7 @@ Flag           | Env Variable           | Default       | Description
 `--addr`        | `SNS_FORWARDER_ADDRESS`     | `:9087`            | Address on which to listen.
 `--debug`       | `SNS_FORWARDER_DEBUG`       | `false`            | Debug mode
 `--arn-prefix`  | `SNS_FORWARDER_ARN_PREFIX`  | not specified      | Prefix to use for SNS topic ARNs. If not specified, will try to be detected automatically.
-`--sns-subject` | `SNS_SUBJECT`               | `PrometheusAlert`  | Optional parameter to be used as the "Subject" line when the message is delivered to email endpoints.
+`--sns-subject` | `SNS_SUBJECT`               | not specified      | Optional parameter to be used as the "Subject" line when the message is delivered to email endpoints.
 
 ## Customising messages with template