Update qtree.md

qtree.md

@@ -82,7 +82,7 @@ It finally has a list of items and a capacity.
 should take a `boundary` and a capacity `n`. It should have the following
 fields:
 - the `boundary` and the capacity `n`
-- a list of items
+- a list of `Ball`
 - a boolean `divided` telling if the tree has been subdivided
 - its four children: `se`, `sw`, `ne` and `nw`, named after cardinal directions
   and set to `None` as default