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

https://github.com/holtzy/R-graph-gallery/blob/e912fe836cd0e2cfbaab01f3ef3f899e0ea929ed/index.html#L43-L46

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

https://github.com/holtzy/R-graph-gallery/pull/65

[holtzy]

Thanks, I fixed it on my side!