Fix example manifest in README.md making target executable

[valeriyvan]

Make .target as .executableTarget in sample manifest in README.
Without this change it's impossible to run benchmark because there's build error:

error: '########': executable product 'benchmark' expects target 'benchmark' to be executable; an executable target requires a 'main.swift' file

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• 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.
• I've updated the documentation (if appropriate).

[lorentey]

Thank you! 👍

[lorentey]

@swift-ci test