Update contributors.rst

[Rorie-Edmunds]

Lines 201 and 229: Changed occurrence to 1 as both are mandatory [Check logic]

[Rorie-Edmunds]

1. The below note for 5.2 contributor.schemaUri is the only hyperlinked schema URI in the documentation. Suggest to remove the hyperlink for consistency (or ensure all others work).
   Note: The controlled list of allowed identifier schemas is defined at https://vocabulary.raid.org/contributor.schemaUri/215 and is shared across all Registration Agencies.

2. The below note for 5.3.2 contributor.position.schemaUri does not seem very useful without either full citations or links to the relevant documents.
   Note: Controlled list informed by Simon Cox’s Project Ontology, OpenAIRE ‘Project’ guidelines, NIH definitions, ARC definitions, and DataCite Metadata Schema 4.4 Appendix 1 Table 5 ‘Description of contributorType’.