Fix to make flag-type doc tags work

In the current published version (0.6.1), so called `flag` doc tags don't actually work.

This was due to their definition containing `flag` (singular) but the render method checking for `flags` (plural).

Fixed by changing `flag` to `flags`. Decided pluralized was the right way since this supports multiple flags.

You can confirm the fix by using any `flag` type doc tag (e.g., `@async`).