Provide additional details to OSV Schema

With https://github.com/ossf/osv-schema/pull/143, the OSV Schema database prefixes can now be documented with additional information:

- How to contribute
- Source URL
- OSV formatted URL

This issue is to track updating [Schema.md](https://github.com/ossf/osv-schema/blob/21c7653f2e948af769618c3b91b94a54d2935ac1/docs/schema.md#id-modified-fields) with these details.