Fix inconsistent case for `API_KEY` config in docs
@@ -350,8 +350,8 @@ WARNING: Secret tokens only provide any security if your APM Server uses TLS.
[options="header"]
|============
-| Environment | `Config` key | Default | Example
-| `ELASTIC_APM_API_KEY` | `api_key` | `None` | A base64-encoded string
+| Environment | Django/Flask | Default | Example
+| `ELASTIC_APM_API_KEY` | `API_KEY` | `None` | A base64-encoded string
|============
experimental::[]
