Fix bug in speaker notes that broke navigating to next slide (#2634)

Change this selector to use the ~= selector to test if a white space
separated word "prev" or "next" is contained

Fixes a speaker notes bug that did not allow going to the next slide in
the speaker notes.
The reason for that is that the "rel" attribute contained "prev"
respective "next prefetch".

See:
https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors#attrvalue_2

This fixes part of #2004 when going to the right (containing
"prefetch").

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";
       });