Merge pull request #594 from phthhieu/patch-1

Update ch04.md

Author: KtorZ

ch04.md

@@ -118,11 +118,11 @@ Should you prefer to do exercises directly in files using your own editor:
 - clone the repository (`git clone [email protected]:MostlyAdequate/mostly-adequate-guide.git`)
 - go in the *exercises* section (`cd mostly-adequate-guide/exercises`)
 - install the necessary plumbing using [npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm) (`npm install`)
-- complete answers by modifying the files named *exercises\_\** in the corresponding chapter's folder 
+- complete answers by modifying the files named *exercise\_\** in the corresponding chapter's folder 
 - run the correction with npm (e.g. `npm run ch04`)
 
 Unit tests will run against your answers and provide hints in case of mistake. By the by, the
-answers to the exercises are available in files named *answers\_\**.
+answers to the exercises are available in files named *solution\_\**.
 
 #### Let's Practice!