Fix Sphinx warning (missing "setting-envvars" reference).
This fixes the following warning when building the docs (probably from
the commit for issue #14901):
"/Doc/faq/windows.rst:138: WARNING: undefined label: setting-envvars (if the
link has no caption the label must precede a section header)"
diff --git a/Doc/using/windows.rst b/Doc/using/windows.rst
index 742a290..c85afd2 100644
--- a/Doc/using/windows.rst
+++ b/Doc/using/windows.rst
@@ -82,6 +82,8 @@
settings in Windows.
+.. _setting-envvars:
+
Excursus: Setting environment variables
---------------------------------------