Fix typo (facebook#3956)

sever => server

Author: andrwh

packages/react-scripts/scripts/start.js

@@ -95,7 +95,7 @@ checkBrowsers(paths.appPath)
     // Load proxy config
     const proxySetting = require(paths.appPackageJson).proxy;
     const proxyConfig = prepareProxy(proxySetting, paths.appPublic);
-    // Serve webpack assets generated by the compiler over a web sever.
+    // Serve webpack assets generated by the compiler over a web server.
     const serverConfig = createDevServerConfig(
       proxyConfig,
       urls.lanUrlForConfig