Format Invoice state filters as inline code instead of headings

Formatting change to code better aligns the parameters as part
of the request and not section headers of the API docs.

- "payed" is either about knots or something nautical
- Capitalised Harvest
- Dropped all periods because they're sentence fragments
Merge pull request #52 from randomecho/invoice-state-filter

Format Invoice state filters as inline code instead of headings -- Thanks @randomecho!