line { format..} example syntax incorrect.

`codec => { line { format => "custom format: %{message}"}}` throws a config error.  Tested 2.3.2, 2.1.1, and 1.5.4.

Fixes #44

Author: jpcarey

lib/logstash/outputs/file.rb

@@ -13,7 +13,7 @@
 # output {
 #  file {
 #    path => ...
-#    codec => { line { format => "custom format: %{message}"}}
+#    codec => line { format => "custom format: %{message}"}
 #  }
 # }
 class LogStash::Outputs::File < LogStash::Outputs::Base