[AD] small patch adding xref in allegro._tx

[Sven Sandberg <svsa1977@xxxxxxxxxx>]

I thought that since the text refers to that section, it can just as
well have an xref too.
-- 
Sven Sandberg    svsa1977@xxxxxxxxxx    home.student.uu.se/svsa1977

--- allegro._tx	Thu Jan 24 18:42:18 2002
+++ allegro._tx	Thu Jan 24 18:42:24 2002
@@ -5759,7 +5759,7 @@
    name, it's out of date.
 
 @@int @file_exists(const char *filename, int attrib, int *aret);
-@domain.hid exists, file_size, file_time
+@xref File, exists, file_size, file_time
    Checks whether a file matching the given name and attributes (see above) 
    exists, returning non-zero if it does. If aret is not NULL, it will be 
    set to the attributes of the matching file. If an error occurs the system