Fix warning that texcheck complained about.

Doc/api/init.tex

@@ -184,7 +184,7 @@
   variable in the top-level \file{Makefile} and the
   \longprogramopt{prefix} argument to the \program{configure} script
   at build time.  The value is available to Python code as
-  \code{sys.prefix}.  It is only useful on \UNIX.  See also the next
+  \code{sys.prefix}.  It is only useful on \UNIX{}.  See also the next
   function.
 \end{cfuncdesc}