Fix scales option in example

[sylhare]

Description

There's a typo for the scales attribute in the documentation.
The ] should not be there.

I tested it:

    options: {
        scales: {
            'left-y-axis': {
                type: 'linear',
                position: 'left'
            },
            'right-y-axis': {
                type: 'linear',
                position: 'right'
            } // if you add here `]` it fails
        }
    }