Minor change to an index entry.

Doc/ref/ref2.tex

@@ -456,7 +456,7 @@ number and an imaginary number).
 \index{octal literal}
 \index{decimal literal}
 \index{imaginary literal}
-\index{complex literal}
+\index{complex!literal}
 
 Note that numeric literals do not include a sign; a phrase like
 \code{-1} is actually an expression composed of the unary operator