First pass at replacing parent-child metaphor to describe docker links.

Addresses issue #7141

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
In this case, "this" is a pronoun and since it's antecedent is a thing (the command), not a place, it is more correct. I don't think readers will notice or be tripped up by the repetition of the word. It's "sentence furniture" that is unlikely to use a lot of interpretive energy.