Add "alt" attribute to be valid HTML.

sphinx/templates/modindex.html

@@ -46,7 +46,7 @@
    <tr{% if indent %} class="cg-{{ cgroup }}"{% endif %}>
      <td>{% if collapse -%}
        <img src="{{ pathto('_static/minus.png', 1) }}" id="toggle-{{ cgroup }}"
-            class="toggler" style="display: none" />
+            class="toggler" style="display: none" alt="-" />
          {%- endif %}</td>
      <td>{% if indent %}&nbsp;&nbsp;&nbsp;{% endif %}
      {% if fname %}<a href="{{ fname }}">{% endif -%}