fix naming

snippets/difference.md

@@ -0,0 +1,8 @@
+### Array difference
+
+Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values not contained in `b`.
+
+```js
+const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
+// difference([1,2,3], [1,2,4]) -> [3]
+```