fix(docs): correct output in composition api example (#2012)

Updated the example output in the documentation to reflect the correct HTML structure. Changed <p> tags to <h1> tags to align with the intended rendered output:

Previous output: <p>こんにちは、世界</p>
Corrected output: <h1>こんにちは、世界</h1>

Author: pejeio

docs/guide/advanced/composition.md

@@ -82,7 +82,7 @@ The output follows:
 
 ```vue
 <div id="app">
-  <p>こんにちは、世界</p>
+  <h1>こんにちは、世界</h1>
 </div>
 ```