Tiny: valGridline in API doc

[Slidemagic]

In https://gitbrent.github.io/PptxGenJS/docs/api-charts.html, it currently says:

object with properties size (pt), color and style ('solid', 'dash' or 'dot') or 'none' to hide

should be:

object with properties size (pt), color and style ('solid', 'dash' or 'dot' or 'none' to hide)

Thank you for all the hard work with V3