add target placeholder

Author: pfefferle

tests/data/mf2/brid-gy-mastodon.html

@@ -68,8 +68,8 @@
   </span>
 
   <span class="u-category h-card">
-    <data class="p-uid" value="https://notiz.blog/author/matthias-pfefferle/"></data>
-    <a class="p-name u-url" href="https://notiz.blog/author/matthias-pfefferle/">pfefferle</a>
+    <data class="p-uid" value="http://example.com/webmention/target/placeholder"></data>
+    <a class="p-name u-url" href="http://example.com/webmention/target/placeholder">pfefferle</a>
 
 
   </span>