link to GIL as a term

Patch from Jeff Knupp.
diff --git a/Doc/library/hashlib.rst b/Doc/library/hashlib.rst
index 19d3e4c..929d41b 100644
--- a/Doc/library/hashlib.rst
+++ b/Doc/library/hashlib.rst
@@ -40,7 +40,7 @@
 
 .. note::
 
-   For better multithreading performance, the Python GIL is released for
+   For better multithreading performance, the Python :term:`GIL` is released for
    strings of more than 2047 bytes at object creation or on update.
 
 .. note::