Documentation/ABI: correct possessive "its" typos

Correct all uses of "it's" that are meant to be possessive "its".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220801025207.29971-1-rdunlap@infradead.org


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