Fix invalid service deprecation (#43)

Author: dunglas

src/DependencyInjection/MercureExtension.php

@@ -190,7 +190,7 @@ public function load(array $configs, ContainerBuilder $container)
                     ->addArgument(new Reference("$publisherId.traceable.inner"))
                     ->addArgument(new Reference('debug.stopwatch'));
 
-                $this->deprecate($traceablePublisher, 'The "%alias_id%" service alias is deprecated. Use "'.$hubId.'.traceable" instead.');
+                $this->deprecate($traceablePublisher, 'The "%service_id%" service is deprecated. Use "'.$hubId.'.traceable" instead.');
 
                 $traceableHubs[$name] = new Reference("$publisherId.traceable");