Add a comment for the RFC number

There's a comment "Unmarked codes are from RFC 2616" so i guess we need to add an explicit comment, not just git commit message.

Author: miyagawa

lib/HTTP/Status.pm

@@ -18,7 +18,7 @@ my %StatusCode = (
     100 => 'Continue',
     101 => 'Switching Protocols',
     102 => 'Processing',                      # RFC 2518 (WebDAV)
-    103 => 'Early Hints',
+    103 => 'Early Hints',                     # RFC 8297
     200 => 'OK',
     201 => 'Created',
     202 => 'Accepted',