Update Onion Ocean background entry to match how the board is referencing it

Author: nikkiwritescode

src/data/backgrounds.yml

@@ -1170,7 +1170,7 @@
   download:
   - https://nikkums.io/cswt/Backgrounds/mira.background.zip
   mapIcon: mira
-- background: onionocean
+- background: OnionOcean
   name: Onion Ocean
   download:
   - https://nikkums.io/cswt/Backgrounds/onionocean.zip