Update opensessions link in singular.jsonld

[howaskew]

Publishing Checklist

I confirm the following:

• My open data feeds conform to one of the permissable combinations specified in https://developer.openactive.io/publishing-data/data-feeds/types-of-feed
• My open data feeds all pass the OpenActive Validator's model validation (https://validator.openactive.io/)
• My open data feeds all pass the OpenActive Validator's RPDE feed validation (https://validator.openactive.io/rpde)
• My open data feeds include activity list references as specified in https://developer.openactive.io/publishing-data/activity-list-references
• My dataset site(s) each reference a GitHub Issues Board

Maintenance Checklist

Our organisation is committed to:

• Ensuring activity providers are aware of the OpenActive initiative on an ongoing basis
• Providing high quality opportunity data that meets user needs, as specified in OpenActive's Data Quality reporting framework
• Resolving issues that are raised on the GitHub Issues Board(s)