Use single quotes for example code

Author: OskarStark

service_container/tags.rst

@@ -425,7 +425,7 @@ use this, update the compiler::
                 foreach ($tags as $attributes) {
                     $definition->addMethodCall('addTransport', [
                         new Reference($id),
-                        $attributes["alias"]
+                        $attributes['alias']
                     ]);
                 }
             }