Fix number cannot be assigned to string issue in script example (with…

…astro#4421)

Co-authored-by: Chris Swithinbank <[email protected]>

src/content/docs/en/guides/client-side-scripts.mdx

@@ -163,7 +163,7 @@ In this example, we define a new `<astro-heart>` HTML element that tracks how ma
       // Each time the button is clicked, update the count.
 			heartButton.addEventListener('click', () => {
         count++;
-        countSpan.textContent = count;
+        countSpan.textContent = count.toString();
       });
 		}
   }