[DOCS] Rename "Linked Agent" to "Contributors" in metadata documentation page

[aOelschlager]

• If applicable, link to the documentation page relevant to your issue below.
  https://github.com/Islandora/documentation/blob/2ade77008088adec76123fac4c31b2712ee59ac5/docs/user-documentation/metadata.md

• If this is a request for new documentation, where in the existing documentation does this documentation belong?
  

  documentation/docs/user-documentation/metadata.md

  Line 209 in 2ade770

  The Islandora Starter Site includes a Typed Relation field labelled 'Linked Agent' as part of the Repository Item content type, and populates the available relations from the MARC relators list. 

• Describe your documentation request.
  Changes to Linked Agent label in Starter Site need to be reflected in the documentation.

• Which version of islandora are you using?
  "drupal/islandora": "^2.8.1"

• What user roles is this documentation intended for?
  All

• What implementations of islandora does this documentation request apply to?
  Metadata

Any related open or closed issues to this bug report?
Islandora-Devops/islandora-starter-site@0bbfd8e

Screenshots: