Doc: use of overlay_dirs in bootstrap example should use import with dest

[mikemccracken]

Is your feature request related to a problem? Please describe.

https://stackerbuild.io/v0.40.1/user_guide/scratch_image/

at the end we have an example of importing a file and putting it somewhere without use of a run section

because overlay_dirs is confusing and should be replaced with uses of import with dest, we should change this example.

Describe the solution you'd like

make the example use import with dest