Avoid blank lines in zone files

[jsha]

Since the goal of basic.md is not to teach the zone file format, and the semantics of blank lines are non-obvious, I think it's better to avoid them.

This was inspired by seeing the blank lines in the wildcard example and not totally understanding what they meant. After that, I saw that they were indeed explained in the "NS Records" section, but as an aside that I had missed.