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/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 69d6523..28510ac 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -14,7 +14,8 @@
 :mod:`multiprocessing` is a package that supports spawning processes using an
 API similar to the :mod:`threading` module.  The :mod:`multiprocessing` package
 offers both local and remote concurrency, effectively side-stepping the
-:term:`Global Interpreter Lock` by using subprocesses instead of threads.  Due
+:term:`Global Interpreter Lock <global interpreter lock>` by using
+subprocesses instead of threads.  Due
 to this, the :mod:`multiprocessing` module allows the programmer to fully
 leverage multiple processors on a given machine.  It runs on both Unix and
 Windows.