Fixed compose error due to missing key in GTNH example (#3238)

Author: SgtMate

examples/gtnh/docker-compose.yaml

@@ -13,7 +13,7 @@ services:
       GENERIC_PACKS_SUFFIX: .zip
       GENERIC_PACKS_PREFIX: https://downloads.gtnewhorizons.com/ServerPacks/
       # if this isn't true, then the container tries to download the modpack every run
-      : "true"
+      SKIP_GENERIC_PACK_UPDATE_CHECK: "true"
       # Make sure that this matches what is in your pack's startserver bash file
       CUSTOM_JAR_EXEC: "-Xms6G -Xmx6G -Dfml.readTimeout=180 @java9args.txt -jar lwjgl3ify-forgePatches.jar nogui"
     volumes: