fix 'write_image' usage information

The 'flash write_image' command erroneously listed the bank number,
when it actually uses target addresses to do that lookup for the user.

src/flash/nor/tcl.c

@@ -726,7 +726,7 @@ static const struct command_registration flash_exec_command_handlers[] = {
 		.name = "write_image",
 		.handler = &handle_flash_write_image_command,
 		.mode = COMMAND_EXEC,
-		.usage = "<bank> [erase] [unlock] <file> [offset] [type]",
+		.usage = "[erase] [unlock] <file> [offset] [type]",
 		.help = "write an image to flash"
 	},
 	{