if_indextoname(3): Correct the RFC in a comment

RFC 2533 refers to 'A Syntax for Describing Media Feature Sets',
which is wrong since the correct reference should be
RFC 2553 'Basic Socket Interface Extensions for IPv6'.

Obtained from:	OpenBSD
MFC after:	1 week

lib/libc/net/if_indextoname.c

@@ -40,7 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <errno.h>
 
 /*
- * From RFC 2533:
+ * From RFC 2553:
  *
  * The second function maps an interface index into its corresponding
  * name.