Move fmf to singular

[Yasir-Rafique]

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 all pass the OpenActive Test Suite's deep validation (https://developer.openactive.io/publishing-data/data-feeds/testing-feeds#openactive-test-suite)
• 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
• I have checked that the times in the feed startDate and endDate match the times on the website referenced by the url, and can confirm that there are no timezone issues

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)