fix typo

Author: manzoid

underbar.js

@@ -55,7 +55,7 @@ const map = function(obj, callback=identity) {
   return results;
 };
 
-// Return an array of the values o a certain property in the collection.
+// Return an array of the values of a certain property in the collection.
 // E.g. given an array of people objects, return an array of just their ages.
 const pluck = function(obj, key) {
   return map(obj, item => item[key]);