Fix active grandchild link class (#962)

An occurrence of `grandchild` was miss-spelled `grand_child` (!).
That caused omission of the `active` class in the link in the nav panel for all grandchildren.

This bug was discovered using `diff` to compare the generated sites built using v0.4.0.rc1 and commit  4396b6b1c836f354bb7aa7edc1a06a49f137d615 on the fix-nav-performance PR branch.

_includes/nav.html

@@ -111,7 +111,7 @@
                 {%- for grandchild in grandchildren_list -%}
                   {%- unless grandchild.nav_exclude -%}
                   <li class="nav-list-item {% if page.url == grandchild.url %} active{% endif %}">
-                    <a href="{{ grandchild.url | relative_url }}" class="nav-list-link{% if page.url == grand_child.url %} active{% endif %}">{{ grandchild.title }}</a>
+                    <a href="{{ grandchild.url | relative_url }}" class="nav-list-link{% if page.url == grandchild.url %} active{% endif %}">{{ grandchild.title }}</a>
                   </li>
                   {%- endunless -%}
                 {%- endfor -%}