Fix markup of an \indexiii --> only two parameters were given.

Doc/ref/ref6.tex

@@ -512,7 +512,7 @@ restrictions, but programs should not abuse this freedom, as future
 implementations may enforce them or silently change the meaning of the
 program.)
 
-\strong{Hierarchical module names:}\indexiii{hierarchical}{module names}
+\strong{Hierarchical module names:}\indexiii{hierarchical}{module}{names}
 when the module names contains one or more dots, the module search
 path is carried out differently.  The sequence of identifiers up to
 the last dot is used to find a ``package''\index{packages}; the final