bpo-29770: remove outdated PYO related info (GH-590) (GH-612)

diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 195f63f..08dc311 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -532,8 +532,8 @@
 
 .. envvar:: PYTHONDONTWRITEBYTECODE
 
-   If this is set to a non-empty string, Python won't try to write ``.pyc`` or
-   ``.pyo`` files on the import of source modules.  This is equivalent to
+   If this is set to a non-empty string, Python won't try to write ``.pyc``
+   files on the import of source modules.  This is equivalent to
    specifying the :option:`-B` option.