Fix typo in Rust ALM example docs.

[kylc]

Main Changes

Fix a typo in the Rust Augmented Lagrangian example problem. This makes the written equation match the code directly below, and match the derivatives below.

[alphaville]

@kylc Many thanks for spotting this typo. Apologies for the delay. I had to make a minor change to our GitHub Actions configurations - I've move this to #326.