add "gunboat" in api description

[i-peresvetov]

it's written in your api documentation: Available values : lighttank, mediumtank, heavytank, tankdestroyer, spaa, fighter, stormovik, bomber, divebomber, torpedoboat, submarinechaser, minelayer, transport, navalferrybarge, destroyer, torpedogunboat, ship

but actually you have another one - "gunboat"

[Sgambe33]

Thank you for pointing it out!