docs: Update token_auth_activity_update default value to match implementation

Signed-off-by: Julius Härtl <jus@bitgrid.net>

config/config.sample.php

@@ -370,7 +370,7 @@ $CONFIG = [
  * Tokens are still checked every 5 minutes for validity
  * max value: 300
  *
- * Defaults to ``300``
+ * Defaults to ``60``
  */
 'token_auth_activity_update' => 60,