Revised and fixed links on dockerrepo.md

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Fixed a typo

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #6581 from fredlf/copyedit_dockerrepo

Revised and fixed links on dockerrepo.md