build(deps): update ringbuf requirement from 0.3 to 0.4 in /demo

Updates the requirements on [ringbuf](https://gitlab.com/agerasev/ringbuf) to permit the latest version.
- [Commits](https://gitlab.com/agerasev/ringbuf/commits/master)

---
updated-dependencies:
- dependency-name: ringbuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

demo/Cargo.toml

@@ -38,6 +38,6 @@ deep_filter = { path = "../libDF/", default-features = false, features = [
 ] }
 ndarray = "^0.15"
 rubato = "0.15"
-ringbuf = "0.3"
+ringbuf = "0.4"
 env_logger = "0.10"
 log = { version = "0.4", features = ["std"] }