Fix tables in api.md

[ekaitz-zarraga]

Tables don't support paragraphs in markdown or they are not properly displayed in the docs

[brocaar]

Hi @ekaitz-zarraga , thanks, but please note that this page is automatically generated by this Makefile:
https://github.com/chirpstack/chirpstack/blob/master/api/md/Makefile

This means that when I merge this PR it will soon be overwritten again.

If there is an issue with the generated output, then it might be better to open an issue here (the tool that generated the md from protobuf):
https://github.com/pseudomuto/protoc-gen-doc

[ekaitz-zarraga]

@brocaar I know, yeah. I just made the PR to let you know there were errors in your docs, where and why.
Now you can use this information as you wish.

I have no idea about your setup, how things are built and so on, that's why I decided to be as useful as I could with the information I had. Sorry if this not the best, but it's on you now ❤️