fix invalid doc about eventratelimit (kubernetes#8194)

add this to --runimte-config will cause such error:
```
error: unknown api groups eventratelimit.admission.k8s.io
```
I have test that this feature works fine without any confiuration
to --runtime-config.

docs/admin/admission-controllers.md

@@ -143,8 +143,6 @@ enabling this admission controller.
 This admission controller mitigates the problem where the API server gets flooded by
 event requests. The cluster admin can specify event rate limits by:
 
- * Ensuring that `eventratelimit.admission.k8s.io/v1alpha1=true` is included in the
-   `--runtime-config` flag for the API server;
  * Enabling the `EventRateLimit` admission controller;
  * Referencing an `EventRateLimit` configuration file from the file provided to the API
    server's command line flag `--admission-control-config-file`: