bpo-42955: Fix sys.module_names doc (GH-24329)

Replace versionchanged markup with versionadded.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index d536fc9..f187895 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1077,7 +1077,7 @@
 
    See also the :attr:`sys.builtin_module_names` list.
 
-   .. versionchanged:: 3.10
+   .. versionadded:: 3.10
 
 
 .. data:: modules