Update survey deadline in announcement message

Author: fmaussion

_config.yml

@@ -24,7 +24,7 @@ html:
   use_issues_button: true
   use_edit_page_button: true
   announcement: |
-    <p>How do you use OGGM? Tell us by filling <a href="https://forms.office.com/e/ZDBCwj3vFb" target="_blank">this short survey</a> (open until May 17). Thanks for your contribution!</p>
+    <p>How do you use OGGM? Tell us by filling <a href="https://forms.office.com/e/ZDBCwj3vFb" target="_blank">this short survey</a> (open until May 20). Thanks for your contribution!</p>
   extra_footer: |
     <p>
     These notebooks are licensed under a <a href="https://github.com/OGGM/tutorials/blob/master/LICENSE.txt" target="_blank">BSD-3-Clause license</a>.