Fix opengraph metadata

[luxedo]

Currently the previews of the pages is not working:

https://www.opengraph.xyz/url/https%3A%2F%2Fwww.r-graph-gallery.com%2F/

even though there is opengraph metadata in index.html

R-graph-gallery/index.html

Lines 43 to 46 in e912fe8

	<meta property="og:title" content="The R Graph Gallery &#8211; Help and inspiration for R charts">
	<meta property="og:image" content="img/overview_RGG.png">
	<meta property="og:description" content="The R graph gallery displays hundreds of charts made with R, always providing the reproducible code.">
	<meta property="og:site_name" content="The R Graph Gallery">

It seems that the image is is missing the root slash and www:
Today:
https://r-graph-gallery.comimg/overview_RGG.png
Correct
https://www.r-graph-gallery.com/img/overview_RGG.png

[luxedo]

I 've sent a PR trying to fix it but I don't think I can test it

#65

[holtzy]

Thanks, I fixed it on my side!