bpo-40854: PYTHONPLATLIBDIR env var added to 3.9 (GH-20735)

diff --git a/Doc/c-api/init_config.rst b/Doc/c-api/init_config.rst
index c51b157..b7298ba 100644
--- a/Doc/c-api/init_config.rst
+++ b/Doc/c-api/init_config.rst
@@ -443,7 +443,7 @@
       by ``--with-platlibdir``, overrideable by the ``PYTHONPLATLIBDIR``
       environment variable.
 
-      .. versionadded:: 3.10
+      .. versionadded:: 3.9
 
    .. c:member:: int buffered_stdio