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/pkgutil.rst b/Doc/library/pkgutil.rst
index 2066cbb..3b17b9a 100644
--- a/Doc/library/pkgutil.rst
+++ b/Doc/library/pkgutil.rst
@@ -68,7 +68,7 @@
 
 .. class:: ImpLoader(fullname, file, filename, etc)
 
-   :term:`Loader` that wraps Python's "classic" import algorithm.
+   :term:`Loader <loader>` that wraps Python's "classic" import algorithm.
 
    .. deprecated:: 3.3
       This emulation is no longer needed, as the standard import mechanism