MDL-78528 course: Use h2 to avoid skipped headings

Use h2 headings instead of h3 for the activity headings to avoid having
skipped headings on the Default activity completion page.

course/templates/defaultactivitycompletion.mustache

@@ -64,7 +64,7 @@
                             <span class="sr-only">{{#str}} collapsecategory, core, {{formatedname}} {{/str}}</span>
                         </span>
                         <img class="iconlarge activityicon ml-4" src="{{icon}}" alt="" aria-hidden="true">
-                        <h3 class="activityname ml-3 mb-0">{{{formattedname}}}</h3>
+                        <h2 class="activityname ml-3 mb-0">{{{formattedname}}}</h2>
                     </a>
                 </div>
                 <div id="activitycompletioncollapse-{{id}}"