replace EPS twitter with Back Market Github :)

Author: hypha

src/content/sponsors/backmarket/index.md

@@ -6,8 +6,7 @@ industry: "Technology"
 description: Where the world shops refurbished tech.
 socials:
   linkedin: "https://www.linkedin.com/company/back-market/"
-  twitter: "https://twitter.com/EuroPython"
-  github:
+  github: "https://github.com/backmarket-oss"
   discord:
   mastodon: https://mastodon.social/@BackMarketEng
   bluesky: https://bsky.app/profile/backmarketeng.bsky.social