Small typo fix facebook#4217

Author: andriijas

packages/react-scripts/template/src/serviceWorker.js

@@ -33,7 +33,7 @@ export function register(config) {
       const swUrl = `${process.env.PUBLIC_URL}/service-worker.js`;
 
       if (isLocalhost) {
-        // This is running on localhost. Lets check if a service worker still exists or not.
+        // This is running on localhost. Let's check if a service worker still exists or not.
         checkValidServiceWorker(swUrl, config);
 
         // Add some additional logging to localhost, pointing developers to the