Update example.rst

Hi Maintainers!

After copying the example from the read the docks page I got the below error:

Error: While importing "app", an ImportError was raised:

Traceback (most recent call last):
  File "C:\Users\44780\PycharmProjects\muck-around-flask-restx\venv\lib\site-packages\flask\cli.py", line 240, in locate_app
    __import__(module_name)
  File "C:\Users\44780\PycharmProjects\muck-around-flask-restx\app.py", line 4, in <module>
    from werkzeug.contrib.fixers import ProxyFix
ModuleNotFoundError: No module named 'werkzeug.contrib'

Looks like this has been solved in commit e186dc4 for the examples folder but not done in this example. 

This is my first pull request so feedback welcome. 

All the best
Rich

Author: Richard Ellison

doc/example.rst

@@ -7,7 +7,7 @@ Here is a full example of a `TodoMVC <http://todomvc.com/>`_ API.
 
     from flask import Flask
     from flask_restx import Api, Resource, fields
-    from werkzeug.contrib.fixers import ProxyFix
+    from werkzeug.middleware.proxy_fix import ProxyFix
 
     app = Flask(__name__)
     app.wsgi_app = ProxyFix(app.wsgi_app)