typo fix in upgrading guide

Author: lsmith77

UPGRADE-2.1.md

@@ -327,7 +327,7 @@ UPGRADE FROM 2.0 to 2.1
 
  ```
  {% for type, flashMessages in app.session.flashbag.all() %}
- {% for flashMessage in flashMessages) %}
+ {% for flashMessage in flashMessages %}
  <div class="flash-{{ type }}">
  {{ flashMessage }}
  </div>