Correct benchmark typo

[mdznr]

“benchmark” was spelled incorrectly

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.
• [ ] I've added tests covering my changes (if appropriate).
• [ ] I've verified that my change compiles and works correctly, and does not break any existing tests.
• [x] I've updated the documentation (if appropriate).

[lorentey]

@swift-ci test