Fix GetEventCoverImageUrl by removing the GuildId in the Url string (#2577)

src/Discord.Net.Core/CDN.cs

@@ -218,7 +218,7 @@ namespace Discord
         /// <param name="size">The size of the image.</param>
         /// <returns></returns>
         public static string GetEventCoverImageUrl(ulong guildId, ulong eventId, string assetId, ImageFormat format = ImageFormat.Auto, ushort size = 1024)
-            => $"{DiscordConfig.CDNUrl}guild-events/{guildId}/{eventId}/{assetId}.{FormatToExtension(format, assetId)}?size={size}";
+            => $"{DiscordConfig.CDNUrl}guild-events/{eventId}/{assetId}.{FormatToExtension(format, assetId)}?size={size}";
 
         private static string FormatToExtension(StickerFormatType format)
         {