fix(v2): allows to create tabs with only one item (#1934)

* fix(v2): allow to create tabs with only one item

It was not possible to have tabs containing only one tab item, so the code below crashed

```
<Tabs
    defaultValue="SomeFile.js"
    values={[
        { label: "SomeFile.js", value: "SomeFile.js" }
    ]}
>
<TabItem value="SomeFile.js">
    Tab content
</TabItem>
</Tabs>
```

* Update index.js

packages/docusaurus-theme-classic/src/theme/Tabs/index.js

@@ -31,7 +31,7 @@ function Tabs(props) {
         ))}
       </ul>
       <div className="margin-vert--md">
-        {children.filter(child => child.props.value === selectedValue)[0]}
+        {[...children].filter(child => child.props.value === selectedValue)[0]}
       </div>
     </div>
   );