Update flatten.md

snippets/flatten.md

@@ -2,7 +2,7 @@
 
 Flattens an array.
 
-Using a new array we concatinate it with the spread input array causing a shallow denesting of any contained arrays
+Use a new array and concatenate it with the spread input array causing a shallow denesting of any contained arrays.
 
 ```js
 const flatten = arr => [ ].concat( ...arr );