cmd/present: correct function reference.

A previous change introduced a new way of storing
which page should be synced with the notes, but
used the wrong function name. This corrects the
function reference.

Fixes golang/go#32902

Change-Id: Iefbb189eae12f0d53e46e6f42d3526302515cab3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184737
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

cmd/present/static/notes.js

@@ -54,7 +54,7 @@ function initNotes() {
   slides.src = slidesUrl;
   w.document.body.appendChild(slides);
 
-  var curSlide = parseInt(localStorage.getItem(getDestSlideKey()), 10);
+  var curSlide = parseInt(localStorage.getItem(destSlideKey()), 10);
   var formattedNotes = '';
   var section = sections[curSlide - 1];
   // curSlide is 0 when initialized from the first page of slides.
@@ -107,7 +107,7 @@ function updateNotes() {
   // When triggered from parent window, notesWindow is null
   // The storage event listener on notesWindow will update notes
   if (!notesWindow) return;
-  var destSlide = parseInt(localStorage.getItem(getDestSlideKey()), 10);
+  var destSlide = parseInt(localStorage.getItem(destSlideKey()), 10);
   var section = sections[destSlide - 1];
   var el = notesWindow.document.getElementById('presenter-notes');