[Robot] Add missing json error (#354)

* Add 20029 Error code

* Update DiscordErrorCode.cs

Co-authored-by: Robot
Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Core/DiscordErrorCode.cs

@@ -65,6 +65,7 @@ namespace Discord
         OnlyOwnerAction = 20018,
         AnnouncementEditRatelimit = 20022,
         ChannelWriteRatelimit = 20028,
+        WriteRatelimitReached = 20029,
         WordsNotAllowed = 20031,
         GuildPremiumTooLow = 20035,
         #endregion