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/memoryview.rst b/Doc/c-api/memoryview.rst
index 77afb02..de429e5 100644
--- a/Doc/c-api/memoryview.rst
+++ b/Doc/c-api/memoryview.rst
@@ -57,7 +57,7 @@
 .. c:function:: Py_buffer *PyMemoryView_GET_BASE(PyObject *mview)
 
    Return either a pointer to the exporting object that the memoryview is based
-   on or *NULL* if the memoryview has been created by one of the functions
+   on or ``NULL`` if the memoryview has been created by one of the functions
    :c:func:`PyMemoryView_FromMemory` or :c:func:`PyMemoryView_FromBuffer`.
    *mview* **must** be a memoryview instance.