Update sample app to be more descriptive

Author: tiangolo

flask/app/main.py

@@ -3,7 +3,8 @@
 
 @app.route("/")
 def hello():
-    return "Hello World from Flask (default)"
+    return "Hello World from Flask in a uWSGI Nginx Docker container with \
+     Python 2.7 (default)"
 
 if __name__ == "__main__":
     app.run(host='0.0.0.0', debug=True, port=80)