Update 11-garbage-collector.md

[noamraph]

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I just learned the chapter on GC, and I want to suggest two changes:

1. The text instructs to remove /nix/var/nix/profiles/default-9-link, however I don't see such files. Instead, it seems that you should remove ~/.local/state/nix/profiles/default-9-link.
2. The command that should remove indirect roots generated by nix-build seems way too broad. It will remove all profiles as well. I wrote another command, which just removes the outputs of nix-build.

For (1), I also had to update the description.

Can you review, and say what you think?

Thanks, Noam