Fix comment typo (#6036)

Author: shawtung

packages/react-scripts/config/webpack.config.js

@@ -249,7 +249,7 @@ module.exports = function(webpackEnv) {
         chunks: 'all',
         name: false,
       },
-      // Keep the runtime chunk seperated to enable long term caching
+      // Keep the runtime chunk separated to enable long term caching
       // https://twitter.com/wSokra/status/969679223278505985
       runtimeChunk: true,
     },