Update multibuild concurrency by kpenfou...

[sour gazelle]

I think this part needs more clarification from
https://docs.dagger.io/sdk/go/959738/get-started

Since errgroup was removed as an example, whoever stumbles upon this page and reaches this Tip section will get confused as to where was errgroup was mentioned in code and how the parallelization works in the go example with dagger

[primal agate]

Good point!