pstore/blk: update the command line example

Use the human readable device name instead of the device number, and
add the required best_effort parameter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: WeiXiong Liao <gmpy.liaowx@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201016132047.3068029-3-hch@lst.de