Update return_type.md

Author: mtshiba

crates/ty_python_semantic/resources/mdtest/function/return_type.md

@@ -383,6 +383,15 @@ def divergent2(value):
 
 reveal_type(divergent2((1,)))  # revealed: tuple[Divergent] | list[Divergent] | None
 
+def list_int(x: int):
+    if x > 0:
+        return list1(list_int(x - 1))
+    else:
+        return list1(x)
+
+# TODO: should be `list[int]`
+reveal_type(list_int(1))  # revealed: list[Divergent] | list[int]
+
 def tuple_obj(cond: bool):
     if cond:
         x = object()