Fixes a speaker notes bug that did not allow going to the right in th…

…e speaker notes.

The reason for that is that the "rel" attribute contained "prev" respective "next prefetch".
Change this selector to use the ~= selector
See: https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors#attrvalue_2

theme/speaker-notes.js

@@ -189,7 +189,7 @@
 
     // Update prev/next buttons to keep speaker note state.
     document
-      .querySelectorAll('a[rel="prev"], a[rel="next"]')
+      .querySelectorAll('a[rel~="prev"], a[rel~="next"]')
       .forEach((elem) => {
         elem.href += "#speaker-notes-open";
       });