Fix configuration validation documentation

[mtausig]

Description

Using k0s validate config, as previously done, produces the warning message

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

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

$ k0s validate config --config k0sctl.yaml       
Command "config" is deprecated, use 'k0s config validate' instead
Error: failed to read config: error unmarshaling JSON: while decoding JSON: json: unknown field "hosts"
                                                                                                                                                        
$ k0s config validate --config k0sctl.yaml 
Error: failed to read config: error unmarshaling JSON: while decoding JSON: json: unknown field "hosts"

Checklist:

• My commit messages are signed-off
• I have checked my code and corrected any misspellings

[twz123]

Thanks!