Fix "duplicated mapping key"

Author: shaedrich

topics/hackathon/index.md

@@ -1,8 +1,7 @@
 ---
 topic: hackathon
-aliases: hackfest, codefest, hackday
+aliases: hackfest, codefest, hackday, hackathons
 display_name: Hackathon
-aliases: hackathons
 related: game-jam, hacktoberfest, game-off, hackathon-kit, hackathon-organiser
 short_description: A hackathon is a gathering where developers collaboratively code in an extreme manner over a short period of time.
 wikipedia_url: https://en.wikipedia.org/wiki/Hackathon