markup fix

Doc/ref/ref8.tex

@@ -34,7 +34,7 @@ in the namespace of \module{__main__}.
 \index{interactive mode}
 \refbimodindex{__main__}
 
-Under {\UNIX}, a complete program can be passed to the interpreter in
+Under \UNIX{}, a complete program can be passed to the interpreter in
 three forms: with the \programopt{-c} \var{string} command line option, as a
 file passed as the first command line argument, or as standard input.
 If the file or standard input is a tty device, the interpreter enters