Missing parameter

In the class DotManager of file ctree/visual/dot_manager.py
The `dot_ast_to_file` method passed in a wrong parameter `filename`
```python
    @staticmethod
    def dot_ast_to_file(ast_node, file_name):
        dot_text = ast_node.to_dot()
        dot_output = DotManager.run_dot(dot_text, filename)
```

This will lead to a global variable filename not found error. 

I fixed this by giving DotManager.run_dot() method the correct parameter.