channelmodes.md: Add newline before second table

Markdown hates things being right after paragraphs without a blank line in between, and as a result decided to eat a table and then throw it back up. Theoretically, adding a newline here should fix it. I don't have a local copy of web-7.0 to test, but I do have some painful experience with markdown tables and also with markdown eating things because of missing newlines.

Please check staff channel scrollback/highlights from around 2019-05-07 18:01UTC and 18:38UTC before committing.