docs: ensure consistency in example for class-and-style.md (#2822)

In the inline style are it was written 13 px instead of 30px for the font size. I re write it to 30px

src/guide/essentials/class-and-style.md

@@ -296,7 +296,7 @@ It is often a good idea to bind to a style object directly so that the template
 ```js
 const styleObject = reactive({
   color: 'red',
-  fontSize: '13px'
+  fontSize: '30px'
 })
 ```