Documentation: Fix 'http' check_command.

Fixes #6379

doc/3-monitoring-basics.md

@@ -31,7 +31,7 @@ Here is an example of a host object which defines two child services:
 
     object Service "http" {
       host_name = "my-server1"
-      check_command = "http_ip"
+      check_command = "http"
     }
 
 The example creates two services `ping4` and `http` which belong to the

itl/command-plugins.conf

@@ -112,7 +112,7 @@ object CheckCommand "http" {
 	import "plugin-check-command"
 
 	command = PluginDir + "/check_http"
-	
+
 	arguments = {
 		"-H" = "$http_vhost$"
 		"-I" = "$http_address$"