Editorial: swap phrasing to indefinite article for Boolean-coercibles.

[DanielRosenwasser]

In the definition for Array.prototype.find, we have

_predicate_ should be a function that accepts three arguments and returns a value that is coercible to a Boolean value.

In several other places like Array.prototype.findIndex, we have

_predicate_ should be a function that accepts three arguments and returns a value that is coercible to the Boolean value *true* or *false*.

While find is the odd-one-out, I think it uses cleaner language, and this PR makes other locations more consistent.

Alternatively, I can amend the definition of find.