Fix typo in README

Author: jaslina

README.md

@@ -18,7 +18,7 @@ For convenience, we have a live API server running at https://conduit.production
 
 The source code for the backend server (available for Node, Rails and Django) can be found in the [main RealWorld repo](https://github.com/gothinkster/realworld).
 
-If you want to change the API URL to a local server, simply edit `src/js/config/app.contants.js` and change `api` to the local server's URL (i.e. `localhost:3000/api`)
+If you want to change the API URL to a local server, simply edit `src/js/config/app.constants.js` and change `api` to the local server's URL (i.e. `localhost:3000/api`)
 
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