Documentation/x86/boot.rst: Correct the example of SETUP_INDIRECT

struct setup_data.len is the length of data field. In case of
SETUP_INDIRECT, it should be sizeof(setup_indirect).

Signed-off-by: Cao jin <jojing64@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Link: https://lore.kernel.org/r/20210127084911.63438-1-jojing64@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>