fix: use selectingChildrenSelectsParent instead of selectingChildrenSelectsParents

[lukasnys]

While using ember-table, I noticed I was getting a type error when passing selectingChildrenSelectsParent to the t.body component (see Selection Mode docs and usage in code)

I had a quick scan through the code and noticed in the types it was wrongfully defined as selectingChildrenSelectsParents (with a trailing "s").

This PR fixes that.