layouts/article: fix ToC language variable.

Otherwise this isn't shown for English either.

Author: MikeMcQuaid

_layouts/article.html

@@ -9,7 +9,7 @@
 
     <h1 class="alt-h1 lh-condensed text-center mb-3">{{ page.title }}</h1>
     <p class="lead text-center text-gray col-md-8 mx-auto mb-4 position-relative">{{ page.description }}</p>
-    {% if lang == "en" %}
+    {% if page.lang == "en" %}
     <nav class="toc mb-4 mb-md-6">
       <div class="card col-sm-8 col-md-4 col-lg-3 mx-auto">
         <a class="toc-trigger d-block text-center p-3">