MDL-72687 assign: add missing label to change user element

mod/assign/templates/grading_navigation_user_selector.mustache

@@ -31,9 +31,10 @@
     This template uses ajax functionality, so it cannot be shown in the template library.
 }}
 <a href="#previous" data-action="previous-user" aria-label="{{#str}} previoususer, mod_assign {{/str}}" title="{{#str}} previoususer, mod_assign {{/str}}">{{{larrow}}}</a>
-    <span data-region="input-field">
-        <select data-action="change-user" data-currentuserid="{{currentuserid}}" data-assignmentid="{{assignmentid}}" data-groupid="{{groupid}}"></select>
-    </span>
+<span data-region="input-field">
+    <label for="change-user-select" class="sr-only">{{#str}} changeuser, mod_assign {{/str}}</label>
+    <select id="change-user-select" data-action="change-user" data-currentuserid="{{currentuserid}}" data-assignmentid="{{assignmentid}}" data-groupid="{{groupid}}"></select>
+</span>
 <a href="#next" data-action="next-user" aria-label="{{#str}} nextuser, mod_assign {{/str}}" title="{{#str}} nextuser, mod_assign {{/str}}">{{{rarrow}}}</a>
 
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