Bump cortex-m-rt from 0.7.2 to 0.7.3 in /examples/stm32f4-event-printer

Bumps [cortex-m-rt](https://github.com/rust-embedded/cortex-m) from 0.7.2 to 0.7.3.
- [Release notes](https://github.com/rust-embedded/cortex-m/releases)
- [Changelog](https://github.com/rust-embedded/cortex-m/blob/master/CHANGELOG.md)
- [Commits](rust-embedded/cortex-m@v0.7.2...v0.7.3)

---
updated-dependencies:
- dependency-name: cortex-m-rt
  dependency-type: indirect
...

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

Author: dependabot[bot]