Linked Validator in Developer Docs not working

The [developer guide](https://github.com/buildingSMART/IDS/blob/master/Documentation/developer-guide.md) also points to this [online validator](https://www.liquid-technologies.com/online-xsd-validator), which won't work with the latest version of the ids spec on master.

Tested with [ids.xsd](https://github.com/buildingSMART/IDS/blob/master/Development/ids.xsd) and [Simple BIM Examples](https://github.com/buildingSMART/IDS/blob/master/Development/IDS_SimpleBIM_examples.ids).

Maybe we should update the documentation with another tool which works (if this is not a problem with the spec). Anyway I didn't want to download another toolchain just to validate the output of our tool. It would be much appreciated, if we could find a working online validator for the schema.

Might be the same issue as #113.

Start with Documentation/developer-guide.md and reproduce the report using Development/ids.xsd and Development/IDS_SimpleBIM_examples.ids in the linked online validator. Check whether the failure is caused by the validator or the specification, then update the developer guide with a working online validator; done means the linked tool accepts the current schema and examples.

Written by the indexing model from the issue text.