Ensure line codec can be found in example ruby filter (#12042)

Fixes elastic/logstash#12041

Co-authored-by: João Duarte <jsvd@users.noreply.github.com>

lib/pluginmanager/templates/codec-plugin/lib/logstash/codecs/example.rb.erb

@@ -25,7 +25,7 @@ class LogStash::Codecs::<%= classify(plugin_name) %> < LogStash::Codecs::Base
   config :append, :validate => :string, :default => ', Hello World!'
 
   def register
-    @lines = LogStash::Codecs::Line.new
+    @lines = LogStash::Plugin.lookup("codec", "line").new
     @lines.charset = "UTF-8"
   end # def register