Bump stm32f4xx-hal in /examples/stm32f4-single-motor-example

Bumps [stm32f4xx-hal](https://github.com/stm32-rs/stm32f4xx-hal) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/stm32-rs/stm32f4xx-hal/releases)
- [Changelog](https://github.com/stm32-rs/stm32f4xx-hal/blob/master/CHANGELOG.md)
- [Commits](stm32-rs/stm32f4xx-hal@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: stm32f4xx-hal
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

examples/stm32f4-single-motor-example/Cargo.lock

@@ -9,7 +9,7 @@ cortex-m = { version = "0.7", features = ["critical-section-single-core"]}
 cortex-m-rtic = "1.1.4"
 panic-probe = { version = "0.3", features = ["print-defmt"] }
 
-stm32f4xx-hal = { version = "0.20.0", features = ["stm32f401", "rtic1"] }
+stm32f4xx-hal = { version = "0.21.0", features = ["stm32f401", "rtic1"] }
 
 defmt = "0.3.6"
 defmt-rtt = "0.4"