add sr text for show/hide answer button

lms/templates/problem.html

@@ -29,7 +29,7 @@
     <button class="save" data-value="${_('Save')}">${_('Save')}<span class="sr"> ${_("your answer")}</span></button>
     % endif
     % if answer_available:
-    <button class="show"><span class="show-label">${_('Show Answer')}</span> </button>
+    <button class="show"><span class='sr'>${_('Toggle Answer Visibility')}</span><span class="show-label">${_('Show Answer')}</span></button>
     % endif
     % if attempts_allowed :
     <div class="submission_feedback" aria-live="polite">