Bug 23653: Add license information in about.pl

Bug 23653: Add license information in about.pl

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
JD Amended patch
            <p>The included <a href="https://github.com/OAI/OpenAPI-Specification/blob/master/schemas/v2.0/schema.json">api/swagger-v2-schema.json</a> file is licensed under the[-the-] <a href="https://github.com/OAI/OpenAPI-Specification/tree/master/schemas/v2.0">Apache License, Version 2.0</a>, by the <a href="https://www.openapis.org/about">OpenAPI Initiative [-(OAI)</a></p>-]{+(OAI)</a>.</p>+}

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>