Include Header Object in data serialization list

The PR adding Appendix B pre-dated giving the Header Object
its own field tables.  This adds "Header Object" to the list
of relevant Objects along with the Paramter Object and the
Encoding Object.

Author: handrews

versions/3.0.4.md

@@ -3842,7 +3842,7 @@ Serializing typed data to plain text, which can occur in `text/plain` message bo
 Schema Objects validate data based on the [JSON Schema data model](https://datatracker.ietf.org/doc/html/draft-wright-json-schema-00#section-4.2), which only recognizes four primitive data types: strings (which are [only broadly interoperable as UTF-8](https://datatracker.ietf.org/doc/html/rfc7159#section-8.1)), numbers, booleans, and `null`.
 Notably, integers are not a distinct type from other numbers, with `type: integer` being a convenience defined mathematically, rather than based on the presence or absence of a decimal point in any string representation.
 
-The [Parameter Object](#parameterObject) and [Encoding Object](#encodingObject) offer features to control how to arrange values from array or object types.
+The [Parameter Object](#parameterObject), [Header Object](#headerObject) and [Encoding Object](#encodingObject) offer features to control how to arrange values from array or object types.
 They can also be used to control how strings are further encoded to avoid reserved or illegal characters.
 However, there is no general-purpose specification for converting schema-validated non-UTF-8 primitive data types (or entire arrays or objects) to strings.