SortedNotesFolder: Fix binary search

I can't remember why I implemented this myself instead of using a
standard implementation and avoiding idiotic bugs like this.

Fixes #172

lib/core/sorted_notes_folder.dart

@@ -109,7 +109,7 @@ class SortedNotesFolder with NotesFolderNotifier implements NotesFolder {
 
       var r = _sortFunc(_notes[mid], note);
       if (r == 0) {
-        return low;
+        return mid;
       }
 
       if (low == high) {