Update index.md

partial fix #627

draft/spec/index.md

@@ -1363,7 +1363,7 @@ URL: <https://public.ccsds.org/pubs/650x0m2.pdf>
 <https://ocfl.io/draft/implementation-notes>
 
 <span id="ref-pairtree"/>**\[PairTree]** Pairtrees for Object Storage. J. Kunze; M. Haye; E. Hetzner; M. Reyes; C.
-Snavely. 12 August 2008\. URL: <https://confluence.ucop.edu/display/Curation/PairTree>
+Snavely. 12 August 2008\. URL: <https://datatracker.ietf.org/doc/html/draft-kunze-pairtree-01>
 
 <span id="ref-rfc6068"/>**\[RFC6068]** The 'mailto' URI Scheme. M. Duerst; L. Masinter; J. Zawinski. IETF. October 2010.
 Proposed Standard. URL: <https://www.rfc-editor.org/rfc/rfc6068>