Improve grammar of asynchronous iterator glossary entry (GH-8657)

(cherry picked from commit cf2c5e8)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>

Author: miss-islington

Doc/glossary.rst

@@ -123,10 +123,10 @@ Glossary
       :meth:`__aiter__` method.  Introduced by :pep:`492`.
 
    asynchronous iterator
-      An object that implements :meth:`__aiter__` and :meth:`__anext__`
+      An object that implements the :meth:`__aiter__` and :meth:`__anext__`
       methods.  ``__anext__`` must return an :term:`awaitable` object.
-      :keyword:`async for` resolves awaitable returned from asynchronous
-      iterator's :meth:`__anext__` method until it raises
+      :keyword:`async for` resolves the awaitables returned by an asynchronous
+      iterator's :meth:`__anext__` method until it raises a
       :exc:`StopAsyncIteration` exception.  Introduced by :pep:`492`.
 
    attribute