Update index.md

[andyrosa2]

minor grammar fix

will throw -> will throw an error

avoiding jargon

[Josh-Cena]

Thanks, but "throw" is a common verb used on MDN without the "an error" object. While we can certainly fix this sporadic case, we can neither fix all of them nor enforce it as a rule, so we've repeatedly made the decision for this that we should leave the choice to the writer's discretion and accept it as a valid form.