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)
The article "the" here reinforces the idea that we are talking about the two specific containers mentioned above. It helps provide context by referring back to previous information, allowing the reader to move forward in the sentence to new information with more confidence and less effort.