Changed const to let in es6 Array.map example

Both the counter and the result array were constants, which caused a syntax error

Author: skarlinski

manuscript/appendices/01_language_features.md

@@ -275,8 +275,8 @@ In ES6 we could write it as follows:
 
 ```javascript
 function map(cb, values) {
- const ret = [];
- const i, len;
+ let ret = [];
+ let i, len;
 
  for(i = 0, len = values.length; i < len; i++) {
  ret.push(cb(values[i]));