Provide a method to modify the workflow in Workflow Lifecycle Challenge 2

[joelnitta]

The answer to the challenge doesn't specify how to modify the workflow. It should be done in a way that invalidates some targets but doesn't actually break anything.

targets-workshop/episodes/lifecycle.Rmd

Lines 173 to 176 in b471c64

	## Challenge 2: What else can the visualization tell us?
	Modify the workflow in `_targets.R`, then run `tar_visnetwork()` again **without** running `tar_make()`.
	What color indicates that a target is out of date?