Replace NUL with null

For consistency with the page.

Author: vstinner

Doc/c-api/bytes.rst

@@ -8,8 +8,8 @@ Bytes Objects
 These functions raise :exc:`TypeError` when expecting a bytes parameter and
 called with a non-bytes parameter.
 
-A :c:type:`PyBytesObject` allocates an extra trailing NUL byte for
-compatibility with NUL terminated C strings. It is not counted in
+A :c:type:`PyBytesObject` allocates an extra trailing null byte for
+compatibility with null terminated C strings. It is not counted in
 :c:func:`PyBytes_Size` length.
 
 .. index:: pair: object; bytes