Documentation/sphinx: fix typos of "its"

Use "its" for possessive form instead of the contraction "it's".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20211222062354.23224-1-rdunlap@infradead.org


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