Fix index entry and anchor for module.__test__ (GH-136674)

It was "doctest.module attribute". Now it is "module attribute".

Author: serhiy-storchaka

Doc/library/doctest.rst

@@ -311,9 +311,13 @@ Which Docstrings Are Examined?
 The module docstring, and all function, class and method docstrings are
 searched.  Objects imported into the module are not searched.
 
+.. currentmodule:: None
+
 .. attribute:: module.__test__
    :no-typesetting:
 
+.. currentmodule:: doctest
+
 In addition, there are cases when you want tests to be part of a module but not part
 of the help text, which requires that the tests not be included in the docstring.
 Doctest looks for a module-level variable called ``__test__`` and uses it to locate other