Remove non-existent specref citation.

Author: msporny

spec/index.html

@@ -3873,7 +3873,7 @@ <h2>Acknowledgements</h2>
     <li>
       Rachel Arnold, Dave Longley. 2020. RDF Dataset Normalization. Report
       submitted to the W3C Credentials Community Group mailing list.
-      https://lists.w3.org/Archives/Public/public-credentials/2021Mar/att-0220/RDFDatasetCanonicalization-2020-10-09.pdf [[arnold-longley-2020]]
+      https://lists.w3.org/Archives/Public/public-credentials/2021Mar/att-0220/RDFDatasetCanonicalization-2020-10-09.pdf
     </li>
     <li>
       Tomaszuk, D., Głąb, S., Turoboś, F., Pawlik, T., Kuziński, D., & Sopek, M.