Incorrect event registration in RouteSubscriberBase

Problem/Motivation

Getting this warning when running drush

Array
(
    [routing.route_alter] => Array
        (
            [eventName] => onAlterRoutes
        )

)
 [warning] Invalid argument supplied for foreach() RegisterEventSubscribersPass.php:38

Proposed resolution

RouteSubscriberBase::getSubscribedEvents() is registering its event like so
$events[RoutingEvents::ALTER] = 'onAlterRoutes';
Per https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Routing%2... the 'onAlterRoutes' event should be returned in an array.