Remove outdated .pyo reference from msilib docs (GH-4461)

Since f299abdafa0f2b6eb7abae274861b19b361c96bc
the remove_pyc() method no longer tries to
remove .pyo files.
diff --git a/Doc/library/msilib.rst b/Doc/library/msilib.rst
index 3773a5c..83b3d49 100644
--- a/Doc/library/msilib.rst
+++ b/Doc/library/msilib.rst
@@ -398,7 +398,7 @@
 
    .. method:: remove_pyc()
 
-      Remove ``.pyc``/``.pyo`` files on uninstall.
+      Remove ``.pyc`` files on uninstall.
 
 
 .. seealso::