replace herokuapp with example.com (#346)

The Heroku free tier is gone or going away.

We've security-related issues with abandoned herokuapp.com links so removing it
(even though it's not a link here).

Author: Trott

Diff for: README.md

@@ -42,7 +42,7 @@ The bot will try to load a `.env` file at the root of the project if it exists t
 
 ```
 GITHUB_TOKEN=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
-SSE_RELAY=https://hook-relay.herokuapp.com
+SSE_RELAY=https://hook-relay.example.com
 ```
 
 **Note the additional `SSE_RELAY` variable:**