Fix Documentation in all places

This doesn't work as it doesn't contain `main()`
```sh
go get github.com/MontFerret/cli
```

Please update all references to:
```sh
go get github.com/MontFerret/cli/ferret
```
One more problem is, `https://`, isn't allowed:
```
go get https://github.com/MontFerret/cli
```

If it's okay with you, I will send a pull request.

Thank you.