Update _includes/nav.html

Correction needed for use of collections.

Co-authored-by: Kevin Lin <kevinl@cs.uw.edu>

_includes/nav.html

 <ul class="nav-list">
-  {%- assign included_pages = site.html_pages
+  {%- assign included_pages = include.pages
         | where_exp:"item", "item.nav_exclude != true"
         | where_exp:"item", "item.title != nil" -%}