Align example config path with docs

[riptl]

What's in this pull request?

The Dockerfile and README create the example config at client.example.toml.
Running nimiq-client creates client.toml.example.

I picked client.example.toml.

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.

[riptl]

Also, (a slightly late) congrats on the PoS mainnet release. You've all done a fantastic job 🎉

[Eligioo]

LGTM. I agree on that it should end with the .toml extension.

My only concern is how much we should consider this 'breaking'. If somebody would have some kind of automated build/deploy script, it could make stuff kaputt.

[jsdanielh]

LGTM