typo found by steve

Reference: /n/atom/patch/applied/sdlooptypo
Date: Thu Apr 24 00:46:30 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/man/3/sdloop	Thu Apr 24 00:46:30 2014
+++ /sys/man/3/sdloop	Thu Apr 24 00:46:30 2014
@@ -1,6 +1,6 @@
 .TH SDLOOP 3
 .SH NAME
-sdloop, addloop, sdusb \- loopback storage device interface
+sdloop, addloop, usbsd \- loopback storage device interface
 .SH SYNOPSIS
 .nf
 .B bind -a #S /dev