USB: g_mass_storage: fixed module name in Kconfig

The Kconfig help message for Mass Storage Gadget claimed the
module will be named "g_file_storage" whereas it should be
"g_mass_storage".

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/Kconfig

@@ -747,7 +747,7 @@ config USB_MASS_STORAGE
 	  which may be used with composite framework.
 
 	  Say "y" to link the driver statically, or "m" to build
-	  a dynamically linked module called "g_file_storage".  If unsure,
+	  a dynamically linked module called "g_mass_storage".  If unsure,
 	  consider File-backed Storage Gadget.
 
 config USB_G_SERIAL