Fix public mixins template text in category sidebar (#2613)

Author: srawlins

lib/templates/html/_sidebar_for_category.html

@@ -7,7 +7,7 @@
   {{/self.hasPublicLibraries}}
 
   {{#self.hasPublicMixins}}
-  <li class="section-title"><a href="{{{library.href}}}#mixins">Mixins</a></li>
+  <li class="section-title"><a href="{{{self.href}}}#mixins">Mixins</a></li>
   {{#self.publicMixinsSorted}}
   <li>{{{ linkedName }}}</li>
   {{/self.publicMixinsSorted}}