fix confusing syntax on set grid method

[ArielMejiaDev]

it fixes an issue related to a pour readable method.

Charts by default would work without grid.

To add a default grid (gray and some opacity... plays really well with Tailwindui styles)

php

$chart->areaChart()->setGrid();

To customize the grid color and opacity you can pass params to the grid setter:

php

$chart->areaChart()->setGrid('#93c5fd', 0.2);