Fix typo in Factories exercise: model --> formula

[milanmlft]

The boot_model() function does not have a model argument. It does have formula.

adv-r/Function-factories.Rmd

Lines 554 to 563 in 1ab8367

	boot_model <- function(df, formula) {
	mod <- lm(formula, data = df)
	fitted <- unname(fitted(mod))
	resid <- unname(resid(mod))
	rm(mod)
	function() {
	fitted + sample(resid)
	}
	}

[hadley]

Thanks!