QLA: Fixed scope on table headers

Fixes CNVS-18336

Test plan:
- Visit a quiz log page
- Navigate through the table via screen reader table navigation commands and
  verify that each non-header cell is read together with its proper header.

Change-Id: I802d3e370b933cae87107678f5c8295b703f2a45
Reviewed-on: https://gerrit.instructure.com/48656
Tested-by: Jenkins
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>

client_apps/canvas_quizzes/apps/events/js/views/session.jsx

@@ -59,14 +59,14 @@ define(function(require) {
 
           <table>
             <tr>
-              <th scope="col">
+              <th scope="row">
                 {I18n.t('session_table_headers.started_at', 'Started at')}
               </th>
               <td>{(new Date(this.props.submission.startedAt)).toString()}</td>
             </tr>
 
             <tr>
-              <th scope="col">
+              <th scope="row">
                 {I18n.t('session_table_headers.attempt', 'Attempt')}
               </th>
               <td>
@@ -100,4 +100,4 @@ define(function(require) {
   });
 
   return Session;
-});
+});