Fix script command in a FAQ entry

In the FAQ section "How do I encode single pictures into movies?", use
-s for generating symbolic links with the ln command.

The script was generating hard links, which is not likely what it was
supposed to do.

Fix issue 2488.
Signed-off-by: Mans Rullgard <mans@mansr.com>

doc/faq.texi

@@ -147,7 +147,7 @@ that match @code{*jpg} to the @file{/tmp} directory in the sequence of
 @file{img001.jpg}, @file{img002.jpg} and so on.
 
 @example
-  x=1; for i in *jpg; do counter=$(printf %03d $x); ln "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
+  x=1; for i in *jpg; do counter=$(printf %03d $x); ln -s "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
 @end example
 
 If you want to sequence them by oldest modified first, substitute