Update threading.rst

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: donbarbos

Doc/library/threading.rst

@@ -21,7 +21,7 @@ where much of the time is spent waiting for external resources.
 Unlike the :mod:`multiprocessing` module, which uses separate processes to
 bypass the :term:`Global Interpreter Lock <global interpreter lock>`, the
 threading module operates within a single process, meaning that all threads
-share the same memory space. However, the GIL limits the performance gains of
+share the same memory space. However, the :term:`GIL` limits the performance gains of
 threading when it comes to CPU-bound tasks, as only one thread can execute
 Python bytecode at a time. Despite this, threading remains a useful tool for
 achieving concurrency in many scenarios.