[3.8] bpo-38600: Change the mark up of NULL in the C API documentation. (GH-16950) (GH-16999)

Replace all *NULL* with ``NULL``.
(cherry picked from commit 25fc088607c855060ed142296dc1bd0125fad1af)
diff --git a/Doc/faq/extending.rst b/Doc/faq/extending.rst
index 2ad2765..aecb56e 100644
--- a/Doc/faq/extending.rst
+++ b/Doc/faq/extending.rst
@@ -89,7 +89,7 @@
 length.  Note that Python bytes objects may contain null bytes so C's
 :c:func:`strlen` should not be used.
 
-To test the type of an object, first make sure it isn't *NULL*, and then use
+To test the type of an object, first make sure it isn't ``NULL``, and then use
 :c:func:`PyBytes_Check`, :c:func:`PyTuple_Check`, :c:func:`PyList_Check`, etc.
 
 There is also a high-level API to Python objects which is provided by the