Point to new collections.abc module for collections ABCs

Doc/library/abc.rst

@@ -20,7 +20,7 @@ regarding a type hierarchy for numbers based on ABCs.)
 
 The :mod:`collections` module has some concrete classes that derive from
 ABCs; these can, of course, be further derived. In addition the
-:mod:`collections` module has some ABCs that can be used to test whether
+:mod:`collections.abc` submodule has some ABCs that can be used to test whether
 a class or instance provides a particular interface, for example, is it
 hashable or a mapping.