made seo image path absolute

Changed image path from relative to absolute - I've included the thumbnail in the site files.

`seo.json` is where the site's metatag info is stored (for detailed display in search results, social media, messengers, etc). The image path was incorrect - it needs to be absolute.

An additional note - you may want to adjust the title and the description for better SEO.
https://metatags.io/ is a good quick way to test how the metatag info will display per platform.
Check out https://github.com/artstorm/eleventy-plugin-seo for more eleventy-seo-plugin settings.

src/_data/seo.json

@@ -4,5 +4,5 @@
     "url": "https://start9.com",
     "author": "Start9 Labs",
     "twitter": "start9labs",
-    "image": "/assets/images/thumbnail.jpg"
-  }
+    "image": "https://start9.com/assets/images/thumbnail.jpg"
+  }