[gitea] Fix incorrect link to swift doc and swift package-registry login command (#29096)
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.
(cherry picked from commit eb5ddc0a78ecfe007a6e279a3c59711cdfb3f701)
