docs: Adapt <ul class="errorlist"> to django output

Author: francoisfreitag

docs/reference.rst

@@ -146,12 +146,12 @@ Usage
     <label for="id_number">
      Number:
     </label>
-    <ul class="errorlist">
+    <ul class="errorlist" id="id_number_error">
      <li>
       Enter a valid phone number (e.g. (506) 234-5678) or a number with an international call prefix.
      </li>
     </ul>
-    <input aria-invalid="true" id="id_number" name="number" required="" type="tel" value="invalid"/>
+    <input aria-describedby="id_number_error" aria-invalid="true" id="id_number" name="number" required="" type="tel" value="invalid"/>
    </div>
 
 .. note:: Because the PhoneNumberField specifies a region, the example number