fix css property

Author: basnijholt

docs/source/_static/custom.css

@@ -1,7 +1,7 @@
 .ignore-css{all:unset !important;}
 
 /* https://gitter.im/pyviz/pyviz?at=5e8efb2b16f84f0461683446 */
-.widget-box.bk form {
+.panel-widget-box.bk form {
     display: none !important;
     margin-left: auto !important;
     margin-right: auto !important;