Fixes a few mistakes in the Swift package registry documentation. Syntax for the `package-registry login` command can be found [here](https://github.com/apple/swift-package-manager/blob/main/Documentation/PackageRegistry/PackageRegistryUsage.md#registry-authentication). I was not sure the best way to compress all of that information, so I just focused on making sure the incorrect `package-registry set` command was fixed.
