doc/manual: Fix @subpage handling

doc/manual: Fix @subpage handling

The subpage "thelist" used to have a title "Pending and Open Tasks"
but the commit c41db358a0100ab changed it to "The List". With
@subpage, it now renders:

    "The List of The List enumerates opportunities for"

instead of

    "The List of Pending and Open Tasks enumerates opportunities for"

This commit fix it to

    "The List enumerates opportunities for"

Change-Id: Ifee0dcd9b3c9f7e651a8748a7afda99eedea3c5c
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: http://openocd.zylin.com/6041
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>