Documentation: admin-guide: correct "it's" to possessive "its"

Correct 2 uses of "it's" to the possessive "its" as needed.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: linux-xfs@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230703232024.8069-1-rdunlap@infradead.org