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

Replace all *NULL* with ``NULL``.
diff --git a/Doc/c-api/gen.rst b/Doc/c-api/gen.rst
index 8d54021..e4b8d25 100644
--- a/Doc/c-api/gen.rst
+++ b/Doc/c-api/gen.rst
@@ -22,23 +22,23 @@
 
 .. c:function:: int PyGen_Check(PyObject *ob)
 
-   Return true if *ob* is a generator object; *ob* must not be *NULL*.
+   Return true if *ob* is a generator object; *ob* must not be ``NULL``.
 
 
 .. c:function:: int PyGen_CheckExact(PyObject *ob)
 
-   Return true if *ob*'s type is *PyGen_Type*; *ob* must not be *NULL*.
+   Return true if *ob*'s type is *PyGen_Type*; *ob* must not be ``NULL``.
 
 
 .. c:function:: PyObject* PyGen_New(PyFrameObject *frame)
 
    Create and return a new generator object based on the *frame* object.
    A reference to *frame* is stolen by this function. The argument must not be
-   *NULL*.
+   ``NULL``.
 
 .. c:function:: PyObject* PyGen_NewWithQualName(PyFrameObject *frame, PyObject *name, PyObject *qualname)
 
    Create and return a new generator object based on the *frame* object,
    with ``__name__`` and ``__qualname__`` set to *name* and *qualname*.
    A reference to *frame* is stolen by this function.  The *frame* argument
-   must not be *NULL*.
+   must not be ``NULL``.