README.md: fix typo
README.md: fix typo

Signed-off-by: Luca Ceresoli <a05bd890c4868ea1807f8564055d1fba77c6ba81@lucaceresoli.net>

lucaceresoli/imx_usb_loader,Adellica/imx_usb_loader,boundarydevices/imx_usb_loader,pacificIT/imx_usb_loader,bwalle/imx_usb_loader,vstehle/imx_usb_loader