bpo-29898: Fix incorrect env variable name (GH-1576)

It should read PYTHONLEGACYWINDOWSSTDIO as stated
in section "Add legacy mode" in PEP 528.

(cherry picked from commit 87fa8a780e9045a26c735f085c07bba4b2d0be60)
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 08dc311..40a06b9 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -571,7 +571,7 @@
 
    .. versionchanged:: 3.6
       On Windows, the encoding specified by this variable is ignored for interactive
-      console buffers unless :envvar:`PYTHONLEGACYWINDOWSIOENCODING` is also specified.
+      console buffers unless :envvar:`PYTHONLEGACYWINDOWSSTDIO` is also specified.
       Files and pipes redirected through the standard streams are not affected.
 
 .. envvar:: PYTHONNOUSERSITE
@@ -700,7 +700,7 @@
    .. versionadded:: 3.6
       See :pep:`529` for more details.
 
-.. envvar:: PYTHONLEGACYWINDOWSIOENCODING
+.. envvar:: PYTHONLEGACYWINDOWSSTDIO
 
    If set to a non-empty string, does not use the new console reader and
    writer. This means that Unicode characters will be encoded according to