[merge demo] Prepend link to diff-match-patch.js with "https:"

demo/merge.html

@@ -11,7 +11,7 @@
 <script src="../mode/css/css.js"></script>
 <script src="../mode/javascript/javascript.js"></script>
 <script src="../mode/htmlmixed/htmlmixed.js"></script>
-<script src="//cdnjs.cloudflare.com/ajax/libs/diff_match_patch/20121119/diff_match_patch.js"></script>
+<script src="https://cdnjs.cloudflare.com/ajax/libs/diff_match_patch/20121119/diff_match_patch.js"></script>
 <script src="../addon/merge/merge.js"></script>
 <style>
     .CodeMirror { line-height: 1.2; }