General cleanup of the builder.md file

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Here is my take:

~~~
**Docker can build images automatically** by reading the instructions form a `Dockerfile`.
A `Dockerfile` is a simple text document that contains all the commands you would normally
execute manually in order to build a Docker image. By calling `docker build` from your terminal,
you can have Docker build your image step by step, executing the instructions successively.
  
~~~