fix(docs/README): syntax for editor_toolbar_buttons has to be with double brackets

[mathieu-coingt]

Sample configuration is currently wrong.
In the form type, you need to define configuration for editor_toolbar_buttons with double brackets.

Before :

$builder->add('myField', WysiwygType::class, [
...
'editor_toolbar_buttons' => ['bold', 'italic', 'underline', ...]
])

After :

$builder->add('myField', WysiwygType::class, [
...
'editor_toolbar_buttons' => [['bold', 'italic', 'underline', ...]]
])