Update swiftlang repository link for Package.swift, Package.resolved

[lamtrinhdev]

Summary:

Update links for repositories moved to the swiftlang org on GitHub

Modifications:

• Package.resolved
• Package.swift

Result:

Correct the link:

• https://github.com/apple/swift-docc-plugin => https://github.com/swiftlang/swift-docc-plugin
• https://github.com/apple/swift-docc-symbolkit => https://github.com/swiftlang/swift-docc-symbolkit

[lamtrinhdev]

Dear @mbrandonw , @stephencelis ,

I have a minor update link for swiftlang repository. Could you please help me to review it?

Thanks, Lam