Update participants

PREPARATIONS.md

@@ -51,15 +51,15 @@ Confirmed (but only if seats available):
 
 Not yet confirmed (but with precedence):
 
-- Saeed Katiraei
-- Rick de Leeuw
-- Roel Zinkstok (ONCO)
-- Martijn Eenink (ONCO)
-- Tarek Malas
-- Jonathan Vis
-- Thomas Chin-A-Woeng
-- Sake (?)
-- Niels (?)
+1. Saeed Katiraei
+2. Rick de Leeuw
+3. Roel Zinkstok (ONCO)
+4. Martijn Eenink (ONCO)
+5. Tarek Malas
+6. Jonathan Vis
+7. Thomas Chin-A-Woeng
+8. Sake (?)
+9. Niels (?)
 
 
 ## Course materials