Improve explanation about how to customize Nginx configuration

[DQNEO]

First of all, thank you for the great effort to maintain this marvelous project.

When I first saw this document, I didn't understand the meaning of This can also be accomplished.
I actually misinterpreted it as "Copying the default nginx configuration file from a running container to the host system can also be accomplished" , but the docker file looked doing the opposite.

So I tried to improve some points:

• "copying the default conf" relates to both mounting and building , so I moved it to the head
• It is not necessarily "complex", so I omitted it.

[yosifkit]

Seems ok to me (minus the minor fix to make the markdown formatter happy).

cc the nginx image maintainer: @thresheek to make sure they are ok with content.

[thresheek]

Thanks @yosifkit - I think the change is good.

[DQNEO]

I've fix the markdown format.