rtos: fix error message

Probably a copy&paste error or remainings of some older version.

Change-Id: Ifb81a9a1fe8242f3b114cd0686dd264fbaad4920
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/1123
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>