Use mdash-es consistently in the library docs index (GH-92762)

(cherry picked from commit efb20a97)

Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>

Doc/library/2to3.rst

 .. _2to3-reference:
 
-2to3 - Automated Python 2 to 3 code translation
-===============================================
+2to3 --- Automated Python 2 to 3 code translation
+=================================================
 
 .. sectionauthor:: Benjamin Peterson <benjamin@python.org>
 
@@ -456,8 +456,8 @@ and off individually.  They are described here in more detail.
    ``from future_builtins import zip`` appears.
 
 
-:mod:`lib2to3` - 2to3's library
--------------------------------
+:mod:`lib2to3` --- 2to3's library
+---------------------------------
 
 .. module:: lib2to3
    :synopsis: The 2to3 library