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

(cherry picked from commit d3ea82aaf940167482df1e08d6482de8f2dd8526)

Co-authored-by: Filipe LaĆ­ns <lains@riseup.net>
diff --git a/Doc/library/sysconfig.rst b/Doc/library/sysconfig.rst
index fa18d62..0049b66 100644
--- a/Doc/library/sysconfig.rst
+++ b/Doc/library/sysconfig.rst
@@ -121,7 +121,7 @@
 
    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.