Update disabling-prototype-extensions.md with possible fix for #20465

[johanrd]

See https://github.com/emberjs/ember.js/issues/20465 and https://github.com/ember-cli/ember-cli/blob/master/CHANGELOG.md#v490

Not sure how we usually teach changes like this, and not sure how to automatically propagate these changes into the docs later than 4.9.0

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