fix(README): typo (#78)

Configuration key should be `WEBHOOK_SECRET` without an 'S' before the underscore, not `WEBHOOKS_SECRET`.

Author: igwejk

README.md

@@ -17,7 +17,7 @@ In the Azure Console, navigate to `Function App` -> your app -> `Configuration`
 
 - `APP_ID`
 - `PRIVATE_KEY` (you can encode your key value at https://www.base64encode.org/)
-- `WEBHOOKS_SECRET`
+- `WEBHOOK_SECRET`
 
 Then in your repository settings, create two secrets: