[2.7] Grammar corrections in abc.rst (GH-10525). (GH-10535)

(cherry picked from commit 8e0b05e2)
Co-authored-by: Andrés Delfino <adelfino@gmail.com>

Doc/library/abc.rst

@@ -19,10 +19,10 @@ was added to Python. (See also :pep:`3141` and the :mod:`numbers` module
 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
+ABCs; these can, of course, be further derived. In addition, the
 :mod:`collections` module 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.
+a class or instance provides a particular interface, for example, if it is
+hashable or if it is a mapping.
 
 
 This module provides the following class: