Fix missing Username prop

src/Discord.Net.Core/Entities/Guilds/GuildProperties.cs

@@ -3,7 +3,7 @@ namespace Discord
     /// <summary>
     ///     Properties that are used to modify an <see cref="IGuild" /> with the specified changes.
     /// </summary>
-    /// <see cref="IGuild.ModifyAsync" />
+    /// <see cref="IGuild.ModifyAsync"/>
     public class GuildProperties
     {
         /// <summary>

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -32,7 +32,6 @@ namespace Discord.Rest
                 Icon = args.Icon.IsSpecified ? args.Icon.Value?.ToModel() : Optional.Create<ImageModel?>(),
                 Name = args.Name,
                 Splash = args.Splash.IsSpecified ? args.Splash.Value?.ToModel() : Optional.Create<ImageModel?>(),
-                Username = args.Username,
                 VerificationLevel = args.VerificationLevel
             };