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

diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 3e07972..f91ab02 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -543,7 +543,7 @@
    If this is set to a non-empty string, it overrides the :data:`sys.platlibdir`
    value.
 
-   .. versionadded:: 3.10
+   .. versionadded:: 3.9
 
 
 .. envvar:: PYTHONSTARTUP