Fix reST label for collections ABCs.
The previous markup hijacked the abstract-base-classes glossary entry,
which resulted in the HTML linking to collections.abc when defining the
generic ABC concept. Now the glossary links to the abc module.
diff --git a/Doc/library/abc.rst b/Doc/library/abc.rst
index 2eced45..3e38cb4 100644
--- a/Doc/library/abc.rst
+++ b/Doc/library/abc.rst
@@ -1,3 +1,5 @@
+.. _abstract-base-classes:
+
:mod:`abc` --- Abstract Base Classes
====================================
@@ -12,7 +14,7 @@
--------------
This module provides the infrastructure for defining an :term:`abstract base
-class` (ABCs) in Python, as outlined in :pep:`3119`; see the PEP for why this
+class` (ABC) in Python, as outlined in :pep:`3119`; see the PEP for why this
was added to Python. (See also :pep:`3141` and the :mod:`numbers` module
regarding a type hierarchy for numbers based on ABCs.)