Correct the check list for the ListItem

The checks in the list, which were unckecked, needed spaces in them to
render correctly in the docs.

Author: davep

docs/widgets/list_item.md

@@ -2,8 +2,8 @@
 
 `ListItem` is the type of the elements in a `ListView`.
 
-- [] Focusable
-- [] Container
+- [ ] Focusable
+- [ ] Container
 
 ## Example