Corrected the -b flag in the readme.md
Corrected the -b flag in the readme.md

I missed correcting the description in https://github.com/raspberrypi/usbboot/pull/96/commits/14e28be2280212b07d3d64ea20609d76da8145fe
raspberrypi/usbboot,raspberrypi/usbboot,raspberrypi/usbboot