build(deps): update rubato requirement from 0.14 to 0.15 in /demo

Updates the requirements on [rubato](https://github.com/HEnquist/rubato) to permit the latest version.
- [Release notes](https://github.com/HEnquist/rubato/releases)
- [Commits](HEnquist/rubato@v0.14.0...v0.15.0)

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

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

Author: dependabot[bot]

demo/Cargo.toml

@@ -37,7 +37,7 @@ deep_filter = { path = "../libDF/", default-features = false, features = [
   "default-model",
 ] }
 ndarray = "^0.15"
-rubato = "0.14"
+rubato = "0.15"
 ringbuf = "0.3"
 env_logger = "0.10"
 log = { version = "0.4", features = ["std"] }