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)"

Doc/using/windows.rst

@@ -82,6 +82,8 @@ In order to run Python flawlessly, you might have to change certain environment
 settings in Windows.
 
 
+.. _setting-envvars:
+
 Excursus: Setting environment variables
 ---------------------------------------