fix bug

app/templates/exercise_export/exercise_user.html.erb

@@ -222,13 +222,13 @@
                   <% end %>
                   <p class="pbt10 flex-nowrap">
                     <span class="pull-left line-34">答案（填空<%= index+1 %>）：</span>
-                    <span class="null-answer"><%= to_markdown(check_answer) %></span>
+                    <span class="null-answer"><%= to_markdown(check_answer,@request_url) %></span>
                   </p>
                 <% end %>
               <% elsif q_type == 4 %>
                 <% check_answer = (user_answer.present?  ? user_answer.first.answer_text : '--') %>
                 <p class="flex-nowrap">
-                  <span class="null-answer main-height"><%= to_markdown(check_answer) %></span>
+                  <span class="null-answer main-height"><%= to_markdown(check_answer,@request_url) %></span>
                 </p>
               <% else %>
                 <div class="mbt10">