Fix misnamed option in varnish sample config

plugins/inputs/varnish/README.md

@@ -20,7 +20,7 @@ This plugin gathers stats from [Varnish HTTP Cache](https://varnish-cache.org/)
 
    ## Optional name for the varnish instance (or working directory) to query
    ## Usually appened after -n in varnish cli
-   #name = instanceName
+   # instance_name = instanceName
 ```
 
 ### Measurements & Fields:

plugins/inputs/varnish/varnish.go

@@ -48,7 +48,7 @@ var sampleConfig = `
 
   ## Optional name for the varnish instance (or working directory) to query
   ## Usually appened after -n in varnish cli
-  #name = instanceName
+  # instance_name = instanceName
 `
 
 func (s *Varnish) Description() string {