[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/c-api/file.rst b/Doc/c-api/file.rst
index 543dc60..5370c4e 100644
--- a/Doc/c-api/file.rst
+++ b/Doc/c-api/file.rst
@@ -21,9 +21,9 @@
 
    Create a Python file object from the file descriptor of an already
    opened file *fd*.  The arguments *name*, *encoding*, *errors* and *newline*
-   can be *NULL* to use the defaults; *buffering* can be *-1* to use the
+   can be ``NULL`` to use the defaults; *buffering* can be *-1* to use the
    default. *name* is ignored and kept for backward compatibility. Return
-   *NULL* on failure. For a more comprehensive description of the arguments,
+   ``NULL`` on failure. For a more comprehensive description of the arguments,
    please refer to the :func:`io.open` function documentation.
 
    .. warning::