fix(element): fix reference error introduced at 2a28888

Author: hankduan

Diff for: lib/element.js

@@ -260,7 +260,7 @@ ElementArrayFinder.prototype.get = function(index) {
         throw new Error('Index out of bound. Trying to access element at ' +
             'index: ' + index + ', but there are only ' +
             parentWebElements.length + ' elements that match locator ' +
-            elementArrayFinder.locator_.toString());
+            self.locator_.toString());
       }
       return [parentWebElements[i]];
     });