Summit: Change last registration date to Aug 22 (#67)

The Volvo folks said this was early enough for their logistics.

Author: magnusbaeck

summit.html

@@ -47,7 +47,7 @@ <h1 class="section-heading">The practicalities</h1>
               Registration: Please sign up using the
               <a href="https://forms.gle/jepf1nzXxdQHnmim9">
                 registration form
-              </a> no later than August&nbsp;15. There's limited space
+              </a> no later than August&nbsp;22. There's limited space
               available and we may have to close the form before that
               date.
             </li>