fix: update flow template schema $id to production docs domain

[dangrondahl]

The $id in schemas/flow-template.json currently points to https://kosli.mintlify.app/schemas/flow-template.json (the Mintlify staging domain).

Once the docs site is live at https://docs.kosli.com, this should be updated to:

"$id": "https://docs.kosli.com/schemas/flow-template.json"

The reference in template-reference/flow_template.md (the # yaml-language-server: comment and the Editor validation section) will also need to be updated to match.