bpo-40204: Fix reference to terms in the doc (GH-21865)

Sphinx 3 requires to refer to terms with the exact case.

For example, fix the Sphinx 3 warning:

Doc/library/pkgutil.rst:71: WARNING: term Loader not found in case
sensitive match.made a reference to loader instead.
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index c5a7f04..a817408 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -2601,7 +2601,7 @@
 -----------------
 
 An :term:`awaitable` object generally implements an :meth:`__await__` method.
-:term:`Coroutine` objects returned from :keyword:`async def` functions
+:term:`Coroutine objects <coroutine>` returned from :keyword:`async def` functions
 are awaitable.
 
 .. note::
@@ -2626,7 +2626,7 @@
 Coroutine Objects
 -----------------
 
-:term:`Coroutine` objects are :term:`awaitable` objects.
+:term:`Coroutine objects <coroutine>` are :term:`awaitable` objects.
 A coroutine's execution can be controlled by calling :meth:`__await__` and
 iterating over the result.  When the coroutine has finished executing and
 returns, the iterator raises :exc:`StopIteration`, and the exception's