fix: update instructions for invoking pixi run commands

[esteve]

This PR fixes the document on how to build one package, neither --package or -- --package worked for me, but just pixi run build-one ros-humble-rsl actually worked:

…/home/esteve/Projects/Bonsai/RoboStack/ros-humble on  main via 🐍 v3.14.4 via 🧚 v0.67.2 
❯ pixi run build-one --package ros-humble-rsl
Error:   × task 'build-one' received more arguments than expected
  help: use `--` to separate task arguments from extra passthrough arguments


…/home/esteve/Projects/Bonsai/RoboStack/ros-humble on  main via 🐍 v3.14.4 via 🧚 v0.67.2 
❯ pixi run build-one -- --package ros-humble-rsl
✨ Pixi task (build-one): cp ./patch/ros-humble-ros-workspace.*patch ./recipes/ros-humble-ros-workspace/patch/; rattler-build build --recipe ./recipes/ros-humble-ros-workspace/recipe.yaml -m ./conda_build_config.yaml -c https://repo.prefix.dev/conda-forge -c robostack-staging --package ros-humble-rsl: (Build a single package, from the ./recipes dir. Add the `ros-humble-` prefix to the package name, e.g. `pixi build-one ros-humble-ros-workspace`)
cp: could not copy patch/ros-humble-ros-workspace.patch to ./recipes/ros-humble-ros-workspace/patch/: No such file or directory (os error 2)
error: unexpected argument '--package' found

  tip: a similar argument exists: '--package-format'

Usage: rattler-build build --recipe <RECIPES> --variant-config <VARIANT_CONFIG> --channel <CHANNELS> --package-format <PACKAGE_FORMAT>

For more information, try '--help'.

…/home/esteve/Projects/Bonsai/RoboStack/ros-humble on  main via 🐍 v3.14.4 via 🧚 v0.67.2 
❯ 

[traversaro]

fyi @Tobias-Fischer