valid-relative-container-URL-with-fragment string in EPUB 3.3

The term "valid-relative-container-URL-with-fragment string" appears in [6.1.5](https://w3c.github.io/epub-specs/epub33/core/#sec-container-iri) in EPUB 3.3.  I did not immediately realize that this term is specific to EPUB.  I think that adding "ocf" somewhere in this term will make this point clearer.