updates index.md with pretix information

Author: Devaraj-G

index.md

@@ -20,7 +20,7 @@ email: ["ARC.Education+carpentries@UCL.ac.uk"]    # boxed, comma-separated list
 collaborative_notes: https://hackmd.io/Cvs768uRQGmRxudx6GnN6Q # optional: URL for the workshop collaborative notes, e.g. an Etherpad or Google Docs document (e.g., https://pad.carpentries.org/2015-01-01-euphoria)
 eventbrite:           # optional: alphanumeric key for Eventbrite registration, e.g., "1234567890AB" (if Eventbrite is being used)
 tickettailor:         # Optional: url bit that points to tickettailor event "1234567/abc/1100"
-pretix:               # Optional: url bit that points to pretix event "organisation/eventid"
+pretix: "ARC/2026-02-18-geo-R"               # Optional: url bit that points to pretix event "organisation/eventid"
 what3words:           # optional: what3words (https://what3words.com) address of the workshop venue, without leading slashes e.g. "globe.lessening.computers"
 ---