Fix configuration validation documentation

Using `k0s validate config` produces the warning message

> Command "config" is deprecated, use 'k0s config validate' instead


Signed-off-by: Mat <mtausig@users.noreply.github.com>

docs/configuration-validation.md

@@ -3,10 +3,10 @@
 k0s command-line interface has the ability to validate config syntax:
 
 ```shell
-k0s validate config --config path/to/config/file
+k0s config validate --config path/to/config/file
 ```
 
-`validate config` sub-command can validate the following:
+`config validate` sub-command can validate the following:
 
 1. YAML formatting
 2. [SAN addresses](/configuration/#specapi)