Update draft-ietf-httpbis-incremental.md

Co-authored-by: Martin Thomson <[email protected]>

Author: kazuho

draft-ietf-httpbis-incremental.md

@@ -232,8 +232,7 @@ Extra Parameters:
 : none
 
 Recommended HTTP Status Code:
-: 403 if refusing to serve an incremental request, or 504 if refusing an
-  incremental response.
+: 501
 
 Response Only Generated By Intermediaries:
 : true