Documentation: USB: correct possessive "its" usage

Correct uses of "it's" to possessive "its" or "its" to "it's" as needed.
Correct associated grammar in one location.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Link: https://lore.kernel.org/r/20221118231422.14076-1-rdunlap@infradead.org


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