MDL-60493 mod_assign: Remove rogue </span>

In the grading interface (/mod/assign/view.php?id=2&action=grader)
there was a rogue </span> with no opening <span>.  Also changed
<span><form> to <div><form> in included template fixing CiBoT HTML
Validation error.

mod/assign/templates/grading_navigation.mustache

@@ -75,7 +75,6 @@
 {{/duedate}}
 </div>
 
-</span>
 </div>
 
 {{!

mod/assign/templates/grading_navigation_user_selector.mustache

@@ -44,7 +44,7 @@
     </small>
 </span>
 
-<span data-region="configure-filters" id="filter-configuration-{{uniqid}}" class="card card-large p-2">
+<div data-region="configure-filters" id="filter-configuration-{{uniqid}}" class="card card-large p-2">
     <form>
         <span class="row px-3 py-1">
             <label class="text-right w-25 p-2 m-0" for="filter-general-{{uniqid}}">
@@ -81,7 +81,7 @@
         </span>
         {{/hasmarkingworkflow}}
     </form>
-</span>
+</div>
 
 <a href="#" data-region="user-filters" title="{{#str}}changefilters, mod_assign{{/str}}" aria-expanded="false" aria-controls="filter-configuration-{{uniqid}}">
     <span class="accesshide">