Fix invalid HTML (#330)

Merge pull request 330

Author: mcnesium

_includes/footer.html

@@ -1,7 +1,7 @@
 <link id="fa-stylesheet" rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@latest/css/all.min.css">
 
 <footer class="site-footer h-card">
-  <data class="u-url" href="{{ '/' | relative_url }}"></data>
+  <data class="u-url" value="{{ '/' | relative_url }}"></data>
 
   <div class="wrapper">