Deprecation: @label attribute on list

The linear algebra book is throwing this deprecation warning:
```
warning: * PTX:DEPRECATE: (2022-04-25) a "@label" attribute (on a "ul" or "ol" element) has been deprecated and should be replaced by the functionally equivalent "@marker".  We will attempt to honor your request (2 times)
*              located within: "Class Activities" (title)
* --------------
```