docs: fix broken link (custom footer) (#951)

docs/customization.md

@@ -139,7 +139,7 @@ The following includes were made available to you:
 
 `_includes/footer_custom.html`
 
-This content appears at the bottom of every page's main content. More info for this include can be found in the [Configuration - Footer content]({% link docs/configuration.md %}#footer-content).
+This content appears at the bottom of every page's main content. More info for this include can be found in the [Configuration - Footer content]({{ site.baseurl }}{% link docs/configuration.md %}#footer-content).
 
 ### Custom Head