Update migration guide with now-converging repos. (#905)
Update migration guide with now-converging repos. (#905)

Motivation:

swift-nio-http2 and swift-nio-transport-services are now converging.

Modifications:

Updated the migration guide.

Result:

Docs will be up to date.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio