Fix typos: OrderedSet Documentation

[kati-kms]

This PR contains a small fix for a typo in OrderedSet Documentation

I found this typo while testing the OrdedSet Documentation - Set Operations

After fix:

Checklist

• [x] I've read the Contribution Guidelines
• [x] My contributions are licensed under the Swift license.
• [x] I've followed the coding style of the rest of the project.
• [x] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [x] I've added benchmarks covering new functionality (if appropriate).
• [x] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [x] I've updated the documentation if necessary.

rdar://118429565

[lorentey]

@kati-kms Would you mind rebasing this PR on the release/1.0 branch?

I think it would make sense to ship it as part of swift-collections 1.0.6.

(If not, we can still land this; I'll just cherry pick it there myself.)

[lorentey]

@swift-ci test

[kati-kms]

@lorentey I've done rebasing this PR on the release/1.0 branch, so please check it!

[lorentey]

Nice! Thank you 👍

[lorentey]

@swift-ci test

[lorentey]

The macOS failure is sadly expected, it's not related to this PR.