Fix Blueprint example with template_folder, pallets#403.

Author: rduplain

docs/blueprints.rst

@@ -61,7 +61,8 @@ implement a blueprint that does simple rendering of static templates::
     from flask import Blueprint, render_template, abort
     from jinja2 import TemplateNotFound
 
-    simple_page = Blueprint('simple_page', __name__)
+    simple_page = Blueprint('simple_page', __name__,
+                            template_folder='templates')
 
     @simple_page.route('/', defaults={'page': 'index'})
     @simple_page.route('/<page>')