RichMedia: fix a compilation error due to nonexistent variable

No idea why this passed Gitlab CI

lib/pleroma/web/rich_media/parser.ex

@@ -39,7 +39,7 @@ defp get_cached_or_parse(url) do
         {:error, :body_too_large} = e ->
           e
 
-        {:error, {:content_type, _}} ->
+        {:error, {:content_type, _}} = e ->
           e
 
         # The TTL is not set for the errors above, since they are unlikely to change