Update RD.XML for TypeConverterAttribute (dotnet/corefx#31183)

Update RD.XML for TypeConverterAttribute (dotnet/corefx#31183)

`GetDefaultAttribute` will look for a field named `Default` on things and there are code paths through which this is reachable with the `TypeConverterAttribute`.

https://github.com/dotnet/corefx/blob/dotnet/corefx@b996738f877d17b0fe426c75b01f0a8ecdc21e07/src/System.ComponentModel.TypeConverter/src/System/ComponentModel/AttributeCollection.cs#L267-L269

Commit migrated from https://github.com/dotnet/corefx/commit/af4f4610d196459441a760fee6c4f372d06e31ba