It is referring to the HTTP/1/1 specification.
The note is a little bit hidden in the spec but there is section near the top on notation:
2 Notational Conventions
Since many of the definitions and syntax are identical to HTTP/1.1,
this specification only points to the section where they are defined
rather than copying it. For brevity, [HX.Y] is to be taken to refer
to Section X.Y of the current HTTP/1.1 specification (RFC 2068 [2]).
I am guessing you know but just in case - this RFC has been superseded and obsoleted by the later RTSP RFC: https://www.rfc-editor.org/rfc/rfc7826