Backport PR #6483: Trim anchor link in heading cells, fixes #6324

This should fix  #6324, which was really irritating me :smiley: I used `.trim()` as suggested by @minrk.

IPython/html/static/notebook/js/textcell.js

@@ -431,7 +431,7 var IPython = (function (IPython) {
             html = security.sanitize_html(html);
             var h = $($.parseHTML(html));
             // add id and linkback anchor
-            var hash = h.text().replace(/ /g, '-');
+            var hash = h.text().trim().replace(/ /g, '-');
             h.attr('id', hash);
             h.append(
                 $('<a/>')