Minor updates and fixes to the CONTRIBUTING doc

* Fixed some uses of docker v. Docker
* Formatting and line wrapping.
* Spelling errors and grammar fixes.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
For one, this extra "s" seems wrong, and for another, this is just a copy of the exact DCO text at [developercertificate.org](http://developercertificate.org/) for reference purposes.