Added missing **'s to Bool.toString

[Orcolom]

[RobLoach]

Other representations of this don't emphasis the heading.... https://github.com/wren-lang/wren/blob/a33832efcfd86c5ac7c91455373b35924b6c1c73/doc/site/modules/core/num.markdown#tostring

[ChayimFriedman2]

It depends on the CSS. The https://wren.io site does:

But it's on Bool's page! Please fix this.