Singularise AAIB aircraft type

Aircraft types doesn't make sense when we can't pluralise this.
Ticket:
http://trello.com/c/I7egEuW8/505-aaib-reports-change-the-plural-aircraft-types-to-the-singular-aircraft-type
Merge pull request #62 from alphagov/singularise-aircraft-type

Singularise AAIB aircraft type