Doc: Remove uneeded wording in Dockerfile format

"The Dockerfile format is quite simple:"

This wording is both subjective and can be offensive especially in technical writing.
If the reader does not fully understand the concept then they are left feeling inadequate.  It also doesn't really add anything to the actual meat of the document.

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)