bpo-35042: Use the :pep: role where a PEP is specified (#10036)

diff --git a/Doc/library/wsgiref.rst b/Doc/library/wsgiref.rst
index e0f745f..ffca1fc 100644
--- a/Doc/library/wsgiref.rst
+++ b/Doc/library/wsgiref.rst
@@ -739,7 +739,7 @@
 
 .. function:: read_environ()
 
-   Transcode CGI variables from ``os.environ`` to PEP 3333 "bytes in unicode"
+   Transcode CGI variables from ``os.environ`` to :pep:`3333` "bytes in unicode"
    strings, returning a new dictionary.  This function is used by
    :class:`CGIHandler` and :class:`IISCGIHandler` in place of directly using
    ``os.environ``, which is not necessarily WSGI-compliant on all platforms