fix jsx-boolean-value doc

[jeffwillette]

• currently it implies that it should be set to a string, but it needs to be in an array to be valid

[palantirtech]

Thanks for your interest in palantir/tslint-react, @deltaskelta! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.