fix variable name in code. (#1028)

CounterApp to Counter.

Author: hasarm

src/guide/introduction.md

@@ -55,7 +55,7 @@ Vue.createApp(Counter).mount('#counter')
 We have already created our very first Vue app! This looks pretty similar to rendering a string template, but Vue has done a lot of work under the hood. The data and the DOM are now linked, and everything is now **reactive**. How do we know? Take a look at the example below where `counter` property increments every second and you will see how rendered DOM changes:
 
 ```js{8-10}
-const CounterApp = {
+const Counter = {
   data() {
     return {
       counter: 0