Small grammatical correction (#367)

* Small grammatical correction

* Fix another grammatical error for consistency

Author: antonky

scripts/start.js

@@ -265,8 +265,8 @@ detect(DEFAULT_PORT).then(port => {
 
   clearConsole();
   var question =
-    chalk.yellow('Something is already running at port ' + DEFAULT_PORT + '.') +
-    '\n\nWould you like to run the app at another port instead?';
+    chalk.yellow('Something is already running on port ' + DEFAULT_PORT + '.') +
+    '\n\nWould you like to run the app on another port instead?';
 
   prompt(question, true).then(shouldChangePort => {
     if (shouldChangePort) {