Fix new credo warnings

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/workers/build_search.ex

@@ -56,8 +56,6 @@ defmodule Mobilizon.Service.Workers.BuildSearch do
   end
 
   defp get_tags_string(%Event{tags: tags}) do
-    tags
-    |> Enum.map(& &1.title)
-    |> Enum.join(" ")
+    Enum.map_join(tags, " ", & &1.title)
   end
 end