[Bug] Typos

[HLWeil]

• https://github.com/nfdi4plants/ARC-specification/blob/main/ISA-XLSX.md#ontology-annotations
  • [x] In the example, Characteristic instead of Characteristics
  • [x] In the example, PURL contains wrong / instead of _
• https://github.com/nfdi4plants/ARC-specification/blob/main/ISA-XLSX.md#assay
  • [x] In the example Study Assay Measurement Type Term Accession Number should have no Study in the beginning