fix(seo): correct social preview image path inside <meta> tag

[cotes2020]

Type of change

• Bug fix (non-breaking change which fixes an issue)

Issue

When a cross-domain site.img_cdn is set and the default site-wide social image URL (site.social_preview_image) is a local path, the URL of the social preview image inside the SEO tag is incorrect.

Input

# _config.yml
img_cdn: https://chirpy-img.netlify.app
social_preview_image: /commons/devices-mockup.png

Output

<!-- The HTML page source -->
<meta property="og:image" content="https://chirpy.cotes.pagehttps://chirpy.cotes.page/commons/devices-mockup.png">
<meta property="twitter:image" content="https://chirpy.cotes.pagehttps://chirpy-img.netlify.app/commons/devices-mockup.png">

What's changed

This PR corrects the logic of _includes/img-url.html to fix the above issue.