Mark copyright example image as fluid

This preserves the old behaviour after openstreetmap#4732 is applied.

app/views/site/copyright.html.erb

@@ -78,7 +78,7 @@
     </p>
     <p><%= image_tag("attribution_example.png",
                      :alt => t(".legal_babble.attribution_example.alt"),
-                     :border => 0,
+                     :border => 0, :class => "img-fluid",
                      :title => t(".legal_babble.attribution_example.title")) %></p>
 
     <h3><%= t ".legal_babble.more_title_html" %></h3>