add missing quote

Author: lohanidamodar

templates/dart/lib/models.dart.twig

@@ -1,6 +1,6 @@
 library {{ language.params.packageName }}.models;
 
-part src/models/model.dart;
+part 'src/models/model.dart';
 {% for definition in spec.definitions %}
 part 'src/models/{{definition.name | caseSnake}}.dart';
 {% endfor %}