[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/conversion.rst b/Doc/c-api/conversion.rst
index ed101c7..c02fa60 100644
--- a/Doc/c-api/conversion.rst
+++ b/Doc/c-api/conversion.rst
@@ -113,7 +113,7 @@
    *val* is a finite number, an infinite number, or not a number, respectively.
 
    The return value is a pointer to *buffer* with the converted string or
-   *NULL* if the conversion failed. The caller is responsible for freeing the
+   ``NULL`` if the conversion failed. The caller is responsible for freeing the
    returned string by calling :c:func:`PyMem_Free`.
 
    .. versionadded:: 3.1