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/contextvars.rst b/Doc/c-api/contextvars.rst
index a7cde7f..38256a3 100644
--- a/Doc/c-api/contextvars.rst
+++ b/Doc/c-api/contextvars.rst
@@ -60,17 +60,17 @@
 .. c:function:: int PyContext_CheckExact(PyObject *o)
 
    Return true if *o* is of type :c:data:`PyContext_Type`. *o* must not be
-   *NULL*.  This function always succeeds.
+   ``NULL``.  This function always succeeds.
 
 .. c:function:: int PyContextVar_CheckExact(PyObject *o)
 
    Return true if *o* is of type :c:data:`PyContextVar_Type`. *o* must not be
-   *NULL*.  This function always succeeds.
+   ``NULL``.  This function always succeeds.
 
 .. c:function:: int PyContextToken_CheckExact(PyObject *o)
 
    Return true if *o* is of type :c:data:`PyContextToken_Type`.
-   *o* must not be *NULL*.  This function always succeeds.
+   *o* must not be ``NULL``.  This function always succeeds.
 
 
 Context object management functions: