Trim anchor link in heading cells, fixes #6324

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

@@ -418,7 +418,7 define([
             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/>')