fix config_name for generated input plugin

Fixes #5355

lib/pluginmanager/templates/input-plugin/lib/logstash/inputs/example.rb.erb

@@ -9,7 +9,7 @@ require "socket" # for Socket.gethostname
 # This plugin is intented only as an example.
 
 class LogStash::Inputs::<%= classify(plugin_name) %> < LogStash::Inputs::Base
-  config_name "<%= @lugin_name %>"
+  config_name "<%= @plugin_name %>"
 
   # If undefined, Logstash will complain, even if codec is unused.
   default :codec, "plain"