GH-100206: use versionadded for the addition of sysconfig.get_default_scheme (GH-100207)

(cherry picked from commit d3ea82a)

Co-authored-by: Filipe Laíns <lains@riseup.net>

Author: miss-islington

Doc/library/sysconfig.rst

@@ -121,7 +121,7 @@ identifier.  Python currently uses eight paths:
 
    Return the default scheme name for the current platform.
 
-   .. versionchanged:: 3.10
+   .. versionadded:: 3.10
       This function was previously named ``_get_default_scheme()`` and
       considered an implementation detail.