Documentation: remove invalid file characters

[compnerd]

: is not supported on Windows as a valid file name character. Replace
the : with -.

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

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 all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[compnerd]

@swift-ci please test

[lorentey]

👍

[lorentey]

@swift-ci test

[compnerd]

@lorentey - mind merging this change? It actually prevents even cloning the repository on Windows.