doc/syntax: fix frame rate documentation

This patch corrects a mistake in the documentation and aligns the
documentation of frame rates to the values that exist in parseutils.c.
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/syntax.texi

@@ -187,17 +187,17 @@ The following abbreviations are recognized:
 @item pal
 25/1
 @item qntsc
-30000/1
+30000/1001
 @item qpal
 25/1
 @item sntsc
-30000/1
+30000/1001
 @item spal
 25/1
 @item film
 24/1
 @item ntsc-film
-24000/1
+24000/1001
 @end table
 
 @anchor{ratio syntax}