Bad link in RFC 7234

In https://tools.ietf.org/html/rfc7234#section-4.2.2

 "heuristics can only be used on responses without explicit freshness
 whose status codes are defined as cacheable by default (see Section
 6.1 of [RFC7231])"

The link marking "Section 6.1" has a URL of
https://tools.ietf.org/html/rfc7234#section-6.1 -- it should be
https://tools.ietf.org/html/rfc7231#section-6.1 instead.

Will.