Update Swift GitHub repo URL

[timsneath]

Update Swift Testing documentation to refer to the new Swift repo location. This is a purely cosmetic change.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

We're holding off on this until we've moved this repo, at which point we'll update all such links. See also #573.

[grynspan]

Closing this PR—we'll address all these issues in one go once the dust settles on the migration.