Update setup.env.example (netbirdio#1433)

This parameter is required as per the documentation, but was missing in the default example.

infrastructure_files/setup.env.example

@@ -65,6 +65,8 @@ NETBIRD_MGMT_IDP="none"
 # Some IDPs requires different client id and client secret for management api
 NETBIRD_IDP_MGMT_CLIENT_ID=$NETBIRD_AUTH_CLIENT_ID
 NETBIRD_IDP_MGMT_CLIENT_SECRET=""
+# Required when setting up with Keycloak "https://<YOUR_KEYCLOAK_HOST_AND_PORT>/admin/realms/netbird"
+# NETBIRD_IDP_MGMT_EXTRA_ADMIN_ENDPOINT=
 # With some IDPs may be needed enabling automatic refresh of signing keys on expire
 # NETBIRD_MGMT_IDP_SIGNKEY_REFRESH=false
 # NETBIRD_IDP_MGMT_EXTRA_ variables. See https://docs.netbird.io/selfhosted/identity-providers for more information about your IDP of choice.