Update spec URLs for flatMap and flat methods (#5054)
Update spec URLs for flatMap and flat methods (#5054)

Array.prototype.flatMap and Array.prototype.flat are now in the
ECMAScript spec (they were previously in a draft proposal). So this
change updates their associated spec URLs.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data