Remove last_modified from documentation index

The method was removed from RDoc, so this change fixes compatibility
with RDoc 6.13.0+

(The line was also removed from edge sdoc already because the value
displayed doesn't really make since: it's the last_modified time of the
Rails README.md and not anything to do with documentation generation)

Author: skipkayhil

lib/rdoc/generator/template/rails/index.rhtml

@@ -22,7 +22,6 @@
         </h2>
         <ul class="files">
             <li><%= h index.relative_name %></li>
-            <li>Last modified: <%= index.last_modified %></li>
         </ul>
         <% if ENV['HORO_BADGE_VERSION'] %>
             <div id="version-badge"><%= ENV['HORO_BADGE_VERSION'] %></div>