re-order SystemChannelId to match api docs

src/Discord.Net.Rest/API/Common/Guild.cs

@@ -25,8 +25,6 @@ namespace Discord.API
         public bool EmbedEnabled { get; set; }
         [JsonProperty("embed_channel_id")]
         public ulong? EmbedChannelId { get; set; }
-        [JsonProperty("system_channel_id")]
-        public ulong? SystemChannelId { get; set; }
         [JsonProperty("verification_level")]
         public VerificationLevel VerificationLevel { get; set; }
         [JsonProperty("default_message_notifications")]
@@ -43,5 +41,7 @@ namespace Discord.API
         public string[] Features { get; set; }
         [JsonProperty("mfa_level")]
         public MfaLevel MfaLevel { get; set; }
+        [JsonProperty("system_channel_id")]
+        public ulong? SystemChannelId { get; set; }
     }
 }