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

Documentation/x86/boot.rst

@@ -851,7 +851,7 @@ Protocol:	2.09+
     struct setup_data {
       __u64 next = 0 or <addr_of_next_setup_data_struct>;
       __u32 type = SETUP_INDIRECT;
-      __u32 len = sizeof(setup_data);
+      __u32 len = sizeof(setup_indirect);
       __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
         __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
         __u32 reserved = 0;