Accessibility: incorrect use of ARIA role="doc-noteref"

[LouiseSteward]

PR #646 refers.

According to the Digital Publishing WAI-ARIA Module 1.0 documentation, role="doc-noteref" should be applied to a superclass of type link (i.e. the <a> in the footnote ref) rather than to the <sup>.

This error has been picked up using the axe extension for Chrome, as well as using Ace by DAISY on EPUBs.