Rename private attribute _element to _el

Change-Id: I9c7a31bd6844636737ffa6456562b997f5e370a3
diff --git a/dev/js/src/tutorial.js b/dev/js/src/tutorial.js
index ee311dd..7963385 100644
--- a/dev/js/src/tutorial.js
+++ b/dev/js/src/tutorial.js
@@ -61,7 +61,7 @@
 	      d.getElementsByTagName('body')[0].appendChild(div);
 
 	      t._iframe = null;
-	      t._element = div;
+	      t._el = div;
 
 	      // Some fields
 	      t._ql     = d.getElementById("ql-field");
@@ -137,7 +137,7 @@
      */
     show : function () {
       const t = this;
-      const element = t._element;
+      const element = t._el;
       if (element.style.display === 'block')
 	      return;
 
@@ -181,7 +181,7 @@
      * Close tutorial window.
      */
     hide : function () {
-      this._element.style.display = 'none';
+      this._el.style.display = 'none';
     },