[README] Fix broken example using `SourceData`

Tried using the example in the README but quickly discovered that `snowcrash::SourceData` doesn't exist.

```bash
$ git grep SourceData
README.md:snowcrash::SourceData blueprint = R"(
```

This PR changes the use here for `mdp::ByteBuffer` which is what the tests are using. I was thinking that perhaps defining `SourceData` would be better so we're not exposing the implementation details. Let me know if that's preferred and I'll amend the PR.