fix data fetching example

Author: yyx990803

src/examples/src/fetching-data/App/composition.js

@@ -1,11 +1,11 @@
 import { ref, watchEffect } from 'vue'
 
 const API_URL = `https://api.github.com/repos/vuejs/core/commits?per_page=3&sha=`
-const branches = ['master', 'v2-compat']
+const branches = ['main', 'v2-compat']
 
 export default {
   setup() {
-    const currentBranch = ref('master')
+    const currentBranch = ref(branches[0])
     const commits = ref(null)
 
     watchEffect(async () => {

src/examples/src/fetching-data/App/options.js

@@ -2,8 +2,8 @@ const API_URL = `https://api.github.com/repos/vuejs/core/commits?per_page=3&sha=
 
 export default {
   data: () => ({
-    branches: ['master', 'v2-compat'],
-    currentBranch: 'master',
+    branches: ['main', 'v2-compat'],
+    currentBranch: 'main',
     commits: null
   }),
 

src/examples/src/fetching-data/App/template.html

@@ -1,4 +1,4 @@
-<h1>Latest Vue.js Commits</h1>
+<h1>Latest Vue Core Commits</h1>
 <template v-for="branch in branches">
   <input type="radio"
     :id="branch"
@@ -17,4 +17,4 @@ <h1>Latest Vue.js Commits</h1>
     </span>
     at <span class="date">{{ formatDate(commit.author.date) }}</span>
   </li>
-</ul>
+</ul>