Fix data-title attribute for H2 elements in TOC (#884)

I believe the data-title attribute for H2 elements was mistakenly using the parent H1 content as its value, rather than its own content.

source/layouts/layout.erb

@@ -77,7 +77,7 @@ under the License.
               <ul class="toc-list-h2">
                 <% h1[:children].each do |h2| %>
                   <li>
-                    <a href="#<%= h2[:id] %>" class="toc-h2 toc-link" data-title="<%= h1[:content] %>"><%= h2[:content] %></a>
+                    <a href="#<%= h2[:id] %>" class="toc-h2 toc-link" data-title="<%= h2[:content] %>"><%= h2[:content] %></a>
                   </li>
                 <% end %>
               </ul>