network-schema: Add unit to Span.fibreLength description

[duncandewhurst]

Related issues

Closes #254

Description

Merge checklist

• Update the changelog (style guide)
• Run ./manage.py pre-commit to update derivative schema files, reference documentation and examples

If there are changes to network-schema.json, network-package-schema.json, reference/publication_formats/json.md, reference/publication_formats/geojson.md or guidance/publication.md#how-to-publish-large-networks, update the relevant manually authored examples:

• examples/json/:
  • network-package.json
  • api-response.json
  • multiple-networks.json
  • network-embedded.json
  • network-separate-endpoints.json
  • network-separate-files.json
  • nodes-endpoint.json
  • spans-endpoint.json
• examples/geojson/:
  • api-response.geojson
  • multiple-networks.geojson

If you used a validation keyword, type or format that is not already used in the schema:

• Update the list of validation keywords, types or formats in JSON Schema usage.
• Add a field that fails validation against the new keyword, type or format to network-package-invalid.json.
• Check that OFDS CoVE reports an appropriate validation error.

If you added a normative rule that is not encoded in JSON Schema:

• Update the list of other normative rules.
• Add a field that does not conform to the rule to network-package-additional-checks.json.
• Open a new issue to add an additional check to Lib Cove OFDS.

If there are changes to examples/geojson/nodes.geojson or examples/geojson/spans.geojson, check and update the data use examples:

• examples/leaflet/leaflet.ipynb
• examples/qgis/geojson.qgs

[duncandewhurst]

Once #259 is merged, then 0.3-dev can be merged into this branch to resolve the check failure.