Correct the order of suggested syntax for extension imports

According to https://github.com/mitsuhiko/flask/issues/1092#issuecomment-47118613 and https://github.com/mitsuhiko/flask/pull/1085#issuecomment-45466907 , the correct order to attempt to import extensions should be:
1) flask_foo
2) flask.ext.foo
3) flaskext_foo.

docs/extensiondev.rst

@@ -410,8 +410,8 @@ deprecated ``flaskext.foo``.  Flask 0.8 introduces a redirect import
 system that lets uses import from ``flask.ext.foo`` and it will try
 ``flask_foo`` first and if that fails ``flaskext.foo``.
 
-Flask extensions should urge users to import from ``flask.ext.foo``
+Flask extensions should urge users to import from ``flask_foo``
-instead of ``flask_foo`` or ``flaskext_foo`` so that extensions can
+instead of ``flask.ext.foo`` or ``flaskext_foo`` so that extensions can
 transition to the new package name without affecting users.