addRelationship 'elements' => '' incorrect formatting

[wpellas]

I was unable to get anything out of 'elements' until I formatted it as an array.
This code worked:
'elements' => ['featured_image']
to see all the featured images next to the relationship selection.
Not sure if I'm a noob or if this is written incorrectly in the cheat sheet as it has no array specified, only ''.
Spent a good while trying to find the solution so if this can help anyone, here it is!