use rich markup for PEP reference (GH-7939)

it's better to refer to PEP with `pep` role in reStructuredText. It also links to the PEP page.
(cherry picked from commit df748c20)
Co-authored-by: Behzad B. Mokhtari <35877268+perplexionist@users.noreply.github.com>

Doc/tutorial/modules.rst

@@ -249,7 +249,7 @@ Some tips for experts:
   directory.
 
 * There is more detail on this process, including a flow chart of the
-  decisions, in PEP 3147.
+  decisions, in :pep:`3147`.
 
 
 .. _tut-standardmodules: