build(deps): bump tokio from 1.20.3 to 1.20.4 in /plans/example-rust

Bumps [tokio](https://github.com/tokio-rs/tokio) from 1.20.3 to 1.20.4.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](tokio-rs/tokio@tokio-1.20.3...tokio-1.20.4)

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

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