[documentation] Not enough parameters on crop method

[dawadam]

(close it if you want bro)

On documentation for croping method :
https://github.com/oliver-moran/jimp/tree/master/packages/plugin-crop

Exemple doesn't work : image.crop(150, 150);

It make error :

Error: w and h must be numbers at Jimp.throwError

Just add 2 parametter to make the job :

image.crop(50, 50, 150, 150);