README.md: Replace ${PROBE_RUN_CHIP} in code example

[bobmcwhirter]

The example makes it look like using a literal ${PROBE_RUN_CHIP} in the config is viable, while it seems to not actually be.

[Urhengulas]

Hi @bobmcwhirter, Thank you for your PR!

What would you think about the following slightly adapted version of yours? Is it clear enough?

---

Add this line to your Cargo configuration file (.cargo/config) and replace ${PROBE_RUN_CHIP} with the particular --chip value:

[target.'cfg(all(target_arch = "arm", target_os = "none"))']
runner = "probe-run --chip ${PROBE_RUN_CHIP}"

For example, one would use nRF52840_xxAA for the nRF52840 microcontroller.
To list all supported chips run probe-run --list-chips.

[bobmcwhirter]

Sure, works for me!

[Urhengulas]

bors r+

[Urhengulas]

Thank you @bobmcwhirter! 😁

[bors]

Build succeeded:

• test (nightly, macOS-latest)
• test (nightly, ubuntu-latest)
• test (nightly, windows-latest)
• test (stable, macOS-latest)
• test (stable, ubuntu-latest)
• test (stable, windows-latest)