correct wrong package reference (#809)

Discard is not part of the io package, it is part of ioutil

Author: blockloop

website/content/guide/customization.md

@@ -19,7 +19,7 @@ to `DEBUG`.
 `Echo#Logger.SetOutput(io.Writer)` can be used to set the output destination for
 the logger. Default value is `os.Stdout`
 
-To completely disable logs use `Echo#Logger.SetOutput(io.Discard)` or `Echo#Logger.SetLevel(log.OFF)`
+To completely disable logs use `Echo#Logger.SetOutput(ioutil.Discard)` or `Echo#Logger.SetLevel(log.OFF)`
 
 ### Log Level