oidc: improve example configuration

Author: ibodrov

server/plugins/oidc/README.md

@@ -23,6 +23,14 @@ concord-server {
 }
 ```
 
+In this case, callback URLs in Okta must be in the form of
+
+```
+http://localhost:8001/api/service/oidc/callback?client_name=oidc
+```
+
+Note the `client_name=oidc` query parameter, it is required by the plugin.
+
 ### Interactive Login
 
 Configure the Concord Console to use custom logout/login URLs: