Update README.md (facebook#2040)

Author: tijwelch

Diff for: README.md

@@ -92,7 +92,7 @@ You will see the build errors and lint warnings in the console.
 ### `npm test` or `yarn test`
 
 Runs the test watcher in an interactive mode.<br>
-By default, runs tests related to files changes since the last commit.
+By default, runs tests related to files changed since the last commit.
 
 [Read more about testing.](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#running-tests)