doc: usbmon: fix wording "be reading until"

The usbmon documentation uses "be reading until" which is an unusual
wording.  Change it to "read until it" which is more clear.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>