Label "Public" classes as "Essential"

Prior to this change, "Public" classes were labeled as "Public", which
differs from the behavior on atom.io. See screenshot:

https://user-images.githubusercontent.com/2988/60625837-f0bc6d00-9db6-11e9-83f4-0e78c12d96cb.png

This commit resolves that discrepancy. "Public" classes will now be
labeled as "Essential".

lib/filters/api_json_filter.rb

@@ -233,7 +233,7 @@ def visibility_label(data)
       <span
         class="label label-#{visibility_class(data["visibility"])}"
         title="This class is in the #{visibility_class(data["visibility"])} API">
-        #{data["visibility"]}
+        #{visibility(data["visibility"])}
       </span>
     HTML
   end