app: fix syntax error

Fix syntax error in app.py.

Fixes: 5884e19 ("Some pycodestyle cleanups")
Signed-off-by: Johannes Thumshirn <jth@kernel.org>

app.py

@@ -30,7 +30,7 @@ def impressum():
 @app.route('/index.html')
 def index():
     posts = [p for p in pages if "date" in p.meta]
-    sorted_pages = sorted(posts, reverse=True, key=lambda page
+    sorted_pages = sorted(posts, reverse=True, key=lambda page:
                           datetime.strptime(page.meta["date"], "%d %b %y"))
     return render_template('index.html', pages=sorted_pages)