fix tangled repo urls #2

Tangled now redirects `tangled.sh` to `tangled.org` and also redirects
`/slices.network/slices` to `/@slices.network/slices`. Some part of this
causes issues when cloning from the URL in the README.

```
$ git clone https://tangled.sh/slices.network/slices
Cloning into 'slices'...
remote: service unsupported: ''
fatal: unable to access 'https://tangled.sh/slices.network/slices/': The requested URL returned error: 403
```

Updating it to the URL in the tangled UI works as expected.

I also updated the only other usage of `tangled.sh` and
`/slices.network/slices` without the `@` to avoid relying on the
redirect.