User Guide - Balance lists in left-hand nav panel

In the source file for the left-hand navigation panel (geode-book/master_middleman/source/subnavs/geode-subnav.erb, the <li> tags outnumber the </li> tags 709 to 702. Not sure if this manifests as a visible error, but it's error-prone for maintenance going forward. Number of opening tags should match the number of closing tags, as with the pairs "<div> </div>" and "<ul> </ul>".

```

64:subnavs$ grep "<div" geode-subnav.erb | wc -l

       2

65:subnavs$ grep "</div" geode-subnav.erb | wc -l

       2

66:subnavs$ grep "<ul" geode-subnav.erb | wc -l

     125

67:subnavs$ grep "</ul" geode-subnav.erb | wc -l

     125

68:subnavs$ grep "<li" geode-subnav.erb | wc -l

     709

69:subnavs$ grep "</li" geode-subnav.erb | wc -l

     702

```