Fixes issue GitbookIO#37

As per hassleg "It's not clear to me what is being asked for in the HOF exercise. A reference is made to a function named add that doesn't exist; I assume this is a typo and that it should be referring to the function named add1. The bigger question is what is meant by the opposite value of that returned by add1? Minus 1?"

Author: svenkatreddy

functions/higher_order.md

@@ -90,7 +90,7 @@ triple(3); // => 9
 
 ---
 
-Define a function named `negate` that takes `add1` as argument and returns a function, that returns the opposite of the value returned by `add`. (Things get a bit more complicated ;) )
+Define a function named `negate` that takes `add1` as argument and returns a function, that returns the negation of the value returned by `add1`. (Things get a bit more complicated ;) )
 
 ```js
 var add1 = function (x) {