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

diff --git a/Doc/library/abc.rst b/Doc/library/abc.rst
index 7071076..2aba02f 100644
--- a/Doc/library/abc.rst
+++ b/Doc/library/abc.rst
@@ -2,7 +2,7 @@
 ====================================
 
 .. module:: abc
-   :synopsis: Abstract base classes according to PEP 3119.
+   :synopsis: Abstract base classes according to :pep:`3119`.
 
 .. moduleauthor:: Guido van Rossum
 .. sectionauthor:: Georg Brandl