fix sphinx syntax in the doc

Author: vstinner

Doc/c-api/refcounting.rst

@@ -34,8 +34,8 @@ of Python objects.
 
    Set the object *o* reference counter to *refcnt*.
 
-   On ref:`Python build with Free Threading <free-threading-buid>`, if *refcnt*
-   is larger than ``UINT32_MAX``, the object is made :term:`immortal`.
+   On :ref:`Python build with Free Threading <free-threading-buid>`, if
+   *refcnt* is larger than ``UINT32_MAX``, the object is made :term:`immortal`.
 
    This function has no effect on :term:`immortal` objects.