adopt `classname` naming in side panel button shared template

wagtail/admin/templates/wagtailadmin/shared/side_panels/includes/side_panel_button.html

@@ -3,13 +3,13 @@
     Variables this template accepts:
 
     text - The text that shows up on the button
-    classes - custom css classes for styling or js
+    classname - custom css classes for styling
     data_url - A url for javascript to use
     has_toggle - Show toggle icon on the button
     attr - custom attributes for the button
 {% endcomment %}
 {% if text %}
-    <button type="button" class="{{ classes }} w-bg-transparent w-text-14 w-p-0 w-text-secondary hover:w-text-secondary-600 w-inline-flex w-justify-center w-transition" {% if data_url %}data-url="{{ data_url }}" {% endif %}{% if has_toggle %}data-button-with-dropdown-toggle {% endif %}{{ attr }}>
+    <button type="button" class="{{ classname }} w-bg-transparent w-text-14 w-p-0 w-text-secondary hover:w-text-secondary-600 w-inline-flex w-justify-center w-transition" {% if data_url %}data-url="{{ data_url }}" {% endif %}{% if has_toggle %}data-button-with-dropdown-toggle {% endif %}{{ attr }}>
         {{ text }}
         {% if has_toggle %}
             {% icon name='arrow-down' class_name='w-w-4 w-h-4 w-transition motion-reduce:w-transition-none' %}