rtla: fix a example in rtla-timerlat-hist.rst

The following error message is reported when running the example in document.

  # timerlat hist -d 10m -c 0-4 -P d:100us:1ms -p 1ms --no-aa
  Failed to set timerlat period
  Could not apply config

The unit of the period is microsecond, '1ms' cannot be accepted.

  usage: [rtla] timerlat hist [-h] [-q] [-d s] [-D] [-n] [-a us] [-p us] [-i us] [-T us] [-s us] ...
         ...
	  -p/--period us: timerlat period in us
         ...

Also fix another minor missleading comment.

Link: https://lore.kernel.org/lkml/20230919133028.697144-1-xiexiuqi@huaweicloud.com



Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>