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.

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.)
 

Doc/library/collections.abc.rst

@@ -23,7 +23,7 @@ example, whether it is hashable or whether it is a mapping.
 .. versionchanged:: 3.3
    Formerly, this module was part of the :mod:`collections` module.
 
-.. _abstract-base-classes:
+.. _collections-abstract-base-classes:
 
 Collections Abstract Base Classes
 ---------------------------------

Doc/library/collections.rst

@@ -34,7 +34,7 @@ Python's general purpose built-in containers, :class:`dict`, :class:`list`,
 =====================   ====================================================================
 
 .. versionchanged:: 3.3
-   Moved :ref:`abstract-base-classes` to the :mod:`collections.abc` module.
+   Moved :ref:`collections-abstract-base-classes` to the :mod:`collections.abc` module.
    For backwards compatibility, they continue to be visible in this module
    as well.