Update README.md

Fixes #9

README.md

@@ -36,7 +36,7 @@ Additionally, you can specify one of several environment variable (via `-e`) to
 
   * `NGROK_AUTH` - Authentication key for your Ngrok account. This is needed for custom subdomains, custom domains, and HTTP authentication.
   * `NGROK_SUBDOMAIN` - Name of the custom subdomain to use for your tunnel. You must also provide the authentication token.
-  * `NGROK_DOMAIN` - Paying Ngrok customers can specify a custom domain. Only one subdomain or domain can be specified, with the domain taking priority.
+  * `NGROK_HOSTNAME` - Paying Ngrok customers can specify a custom domain. Only one subdomain or domain can be specified, with the domain taking priority.
   * `NGROK_USERNAME` - Username to use for HTTP authentication on the tunnel. You must also specify an authentication token.
   * `NGROK_PASSWORD` - Password to use for HTTP authentication on the tunnel. You must also specify an authentication token.
   * `NGROK_PROTOCOL` - Can either be `HTTP` or `TCP`, and it defaults to `HTTP` if not specified. If set to `TCP`, Ngrok will allocate a port instead of a subdomain and proxy TCP requests directly to your application.