Update functions.Rmd (hadley#293)

Small typo

functions.Rmd

@@ -662,7 +662,7 @@ Figuring out what your function should return is usually straightforward: it's w
 
 ### Explicit return statements
 
-The value returned by the function is the usually the last statement it evaluates, but you can choose to return early by using `return()`. I think it's best to save the use of `return()` to signal that you can return early with a simpler solution. A common reason to do this is because the inputs are empty:
+The value returned by the function is usually the last statement it evaluates, but you can choose to return early by using `return()`. I think it's best to save the use of `return()` to signal that you can return early with a simpler solution. A common reason to do this is because the inputs are empty:
 
 ```{r}
 complicated_function <- function(x, y, z) {