Invalid XML Error - Minor Convenience update

[owlboy]

Hello!

Minor update request:

When you have an incorrect XML file, you helpfully link to where I could get it validated to sort out the issue. Lets take this a step further by making it load the XML in question too!

So, all we need to do is replace the link to the validator site with one with a query string for the URL to the sbplus.xml file of the presentation.

Original:
Validate the XML at <a href="https://validator.w3.org/">https://validator.w3.org/</a>.

Proposed Change:
<a href="https://validator.w3.org/check?uri=$$SBPLUS.XML$$&charset=%28detect+automatically%29&doctype=Inline&group=0">Validate the XML at the WC3 Markup Validation Service</a>
($$SBPLUS.XML$$ is where the URL to the sbplus.xml of the project goes)

All other text and formatting choices that are currently on the page are great.

[Lin87]

This is completed and included in version 3.1.0.