fix canonical ref (#1333)

Author: drammock

doc/_templates/layout.html

@@ -12,7 +12,7 @@
 {% endblock %}
 
 {% block extrahead %}
-    <link rel="canonical" href="https://mne.tools/mne-bids/stable/index.html" />
+    <link rel="canonical" href="https://mne.tools/mne-bids/stable/{{ pagename }}.html" />
     <script type="text/javascript" src="{{ pathto('_static/copybutton.js', 1) }}"></script>
     <script type="text/javascript" src="{{ pathto('_static/scrollfix.js', 1) }}"></script>
 {{ super() }}