Fix a missing pluralization

[kurtmckee]

Related Issue(s): N/A

Proposed Changes:

1. Fixed a missing 's' on the word "providers".

   providers is an array so this sentence only makes sense if the ID is unique across all of the listed providers (plural).

PR Checklist:

• [x] This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• [x] This PR has no breaking changes.
• [x] I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• [ ] This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.

[m-mohr]

Thank you for the PR. We discussed it in the STAC PSC meeting today and came to the conclusion that both "provider" and "providers" is ambiguous and thus we drafted a new wording that hopefully is more clear. See PR #1209 for details.

[kurtmckee]

Thanks Matthias!