Fix RSS feed_route not set

[Simounet]

I made some changes earlier and didn't think about other routes using this. So here is the code to fix this error:

 request.CRITICAL: Uncaught PHP Exception Twig\Error\RuntimeError: "An exception has been thrown during the rendering of a template ("Unable to generate a URL for the named route ".fr" as such route does not exist.")." at /var/www/wallabag/src/Wallabag/CoreBundle/Resources/views/Entry/_feed_link.html.twig line 12 {"exception":"[object] (Twig\\Error\\RuntimeError(code: 0): An exception has been thrown during the rendering of a template (\"Unable to generate a URL for the named route \".fr\" as such route does not exist.\"). at /var/www/wallabag/src/Wallabag/CoreBundle/Resources/views/Entry/_feed_link.html.twig:12, Symfony\\Component\\Routing\\Exception\\RouteNotFoundException(code: 0): Unable to generate a URL for the named route \".fr\" as such route does not exist. at /var/www/wallabag/vendor/symfony/symfony/src/Symfony/Component/Routing/Generator/CompiledUrlGenerator.php:50)"} []