Documentation: Fix size parameter for snprintf

cppcheck reports this error:

Documentation/laptops/hpfall.c:33]: (error)
 Dangerous usage of 'devname' (strncpy doesn't always 0-terminate it)

The terminating '\0' is needed for the global char array unload_heads_path,
so never write the last array entry (which is initially '\0').
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/laptops/hpfall.c

@@ -29,7 +29,7 @@ int set_unload_heads_path(char *device)
 		return -EINVAL;
 	strncpy(devname, device + 5, sizeof(devname));
 
-	snprintf(unload_heads_path, sizeof(unload_heads_path),
+	snprintf(unload_heads_path, sizeof(unload_heads_path) - 1,
 				"/sys/block/%s/device/unload_heads", devname);
 	return 0;
 }