switch popovertoggletarget to popovertarget

Co-authored-by: Kristján Oddsson <koddsson@gmail.com>

Author: keithamus

_includes/header.html

@@ -6,6 +6,6 @@
     <div>Beta</div>
   </div>
   {% include 'global-search.html' %}
-  <button slot="opener" popovertoggletarget="sidebar-nav">{% icon "menu" %}</button>
+  <button slot="opener" popovertarget="sidebar-nav">{% icon "menu" %}</button>
   {% include 'global-navigation.html' %}
 </header>