Change id for primary nav to #primary-nav (#41)

The skip to primary navigation links were not working as they referenced an incorrect anchor ID.

Author: anarchivist

_includes/navigation.html

@@ -1,4 +1,4 @@
-<nav id="nav-primary" class="site-nav" itemscope itemtype="http://schema.org/SiteNavigationElement" aria-label="Main navigation">
+<nav id="primary-nav" class="site-nav" itemscope itemtype="http://schema.org/SiteNavigationElement" aria-label="Main navigation">
   <ul id="menu-main-navigation" class="menu">
     <!-- Home link -->
     <li class="menu-item">
@@ -22,4 +22,4 @@
       {% endif %}
     {% endfor %}
   </ul>
-</nav>
+</nav>