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/iter.rst b/Doc/c-api/iter.rst
index 546efb5..0224d37 100644
--- a/Doc/c-api/iter.rst
+++ b/Doc/c-api/iter.rst
@@ -16,8 +16,8 @@
 
    Return the next value from the iteration *o*.  The object must be an iterator
    (it is up to the caller to check this).  If there are no remaining values,
-   returns *NULL* with no exception set.  If an error occurs while retrieving
-   the item, returns *NULL* and passes along the exception.
+   returns ``NULL`` with no exception set.  If an error occurs while retrieving
+   the item, returns ``NULL`` and passes along the exception.
 
 To write a loop which iterates over an iterator, the C code should look
 something like this::