[Workflow] Update workflow.rst

[noniagriconomie]

Fixes #14637
I do not know if I should also write in the code exemple, feel free to comment related to ⬇️

        // this injects the blog_publishing "workflow" configured before
        // if blog_publishing is of type "state_machine", consider
        // injecting WorkflowInterface $blogPublishingStateMachine
        public function __construct(WorkflowInterface $blogPublishingWorkflow)
        {
            $this->blogPublishingWorkflow = $blogPublishingWorkflow;
        }

[javiereguiluz]

Thank you Antoine.