USB: Documentation/usb/gadget_serial.txt: update to match driver use_acm behaviour

Commit 7bb5ea54 (usb gadget serial: use composite gadget framework)
changed the default for the use_acm parameter from 0 to 1.
Update the documentation to match.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>