Symlink for plymouthd.defaults was pointing to nonexisting target, seemingly
because of a spelling error. I don't know that fixing the symlink actually makes a difference for the service, but /etc/plymouth certainly looks tidier :)

Be gentle, this is my first PR ever.

Motivation for this change

I just had a look around in /etc/plymouth and noticed that plymouthd.defaults was a dangling symlink.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.