add position:absolute style to tooltip example code

[yangdanny97]

the code in the examples is missing this style for the tooltip, so it doesn't work as-is when copied

https://www.reddit.com/r/d3js/comments/1azxo4y/scatterplot_tooltip_not_working_correctly/

the version hosted on the graph gallery website happens to work because the position: absolute style for .tooltip is added by bootstrap, but running the code on a page without bootstrap won't work