Update heapless requirement in /examples/i2s-controller-demo (#444)

Updates the requirements on [heapless](https://github.com/japaric/heapless) to permit the latest version.
- [Changelog](https://github.com/rust-embedded/heapless/blob/main/CHANGELOG.md)
- [Commits](rust-embedded/heapless@v0.7.10...v0.8.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/i2s-controller-demo/Cargo.toml

@@ -15,5 +15,5 @@ embedded-hal = "1.0.0"
 systick-monotonic = "1.0.0"
 rtt-target = { version = "0.3.1", features = ["cortex-m"] }
 nrf52840-hal = { features = ["rt"], path = "../../nrf52840-hal" }
-heapless = "0.7.10"
+heapless = "0.8.0"
 small_morse = "0.1.0"