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/library/importlib.rst b/Doc/library/importlib.rst
index f7286d2..5fb0a4a 100644
--- a/Doc/library/importlib.rst
+++ b/Doc/library/importlib.rst
@@ -1073,7 +1073,7 @@
 
 .. class:: WindowsRegistryFinder
 
-   :term:`Finder` for modules declared in the Windows registry.  This class
+   :term:`Finder <finder>` for modules declared in the Windows registry.  This class
    implements the :class:`importlib.abc.MetaPathFinder` ABC.
 
    Only class methods are defined by this class to alleviate the need for
@@ -1088,7 +1088,7 @@
 
 .. class:: PathFinder
 
-   A :term:`Finder` for :data:`sys.path` and package ``__path__`` attributes.
+   A :term:`Finder <finder>` for :data:`sys.path` and package ``__path__`` attributes.
    This class implements the :class:`importlib.abc.MetaPathFinder` ABC.
 
    Only class methods are defined by this class to alleviate the need for