Fix installation instruction

Closes codemirror#4333

Author: marijnh

README.md

@@ -30,5 +30,5 @@ conduct.
 ### Quickstart
 
 To build the project, make sure you have Node.js installed (at least version 6)
-and then `npm install && npm build`. To run, just open `index.html` in your
+and then `npm install && npm run build`. To run, just open `index.html` in your
 browser (you don't need to run a webserver). Run the tests with `npm test`.