[documentation] Not enough parameters on crop method

(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);`