Documentation: Fix typo in proc_usb_info.txt

Correct typo "filsystem" to "filesystem" in
Documentation/usb/proc_usb_info.txt
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Documentation/usb/proc_usb_info.txt

@@ -7,7 +7,7 @@ The usbfs filesystem for USB devices is traditionally mounted at
 /proc/bus/usb.  It provides the /proc/bus/usb/devices file, as well as
 the /proc/bus/usb/BBB/DDD files.
 
-In many modern systems the usbfs filsystem isn't used at all.  Instead
+In many modern systems the usbfs filesystem isn't used at all.  Instead
 USB device nodes are created under /dev/usb/ or someplace similar.  The
 "devices" file is available in debugfs, typically as
 /sys/kernel/debug/usb/devices.