Correct a typo in flask example (#1421)

Author: ngocmphan

docs/flask.rst

@@ -3,7 +3,7 @@ Using folium with flask
 
 A very common use case is to use folium with in a flask app.
 The trick is to return folium's HTML representation.
-Here is an example on how to d that:
+Here is an example on how to do that:
 
 
 .. literalinclude:: ../examples/flask_example.py