usb documentation: fix typo

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
diff --git a/doc/README.usb b/doc/README.usb
index ef1d6ba..b4c3ef5 100644
--- a/doc/README.usb
+++ b/doc/README.usb
@@ -63,7 +63,7 @@
 Storage USB Commands:
 - usb scan:	    scans the USB for storage devices.The USB must be
 		    running for this command (usb start)
-- usb device [dev]: show or set current USB staorage device
+- usb device [dev]: show or set current USB storage device
 - usb part [dev]:   print partition table of one or all USB storage
 		    devices
 - usb read addr blk# cnt: