Grammar fixes in help text

Co-authored-by: Michael Schmidt <[email protected]>

index.html

@@ -202,7 +202,7 @@ <h2 id="basic-usage-cdn">Usage with CDNs</h2>
 <code>&lt;/body>
 &lt;/html></code></pre>
 
-	<p>Please note that links in the above code sample serves as a placeholder. You would have to replace them with valid links to CDNs of your choice.</p>
+	<p>Please note that links in the above code sample serve as placeholders. You have to replace them with valid links to the CDN of your choice.</p>
 
 	<p>CDNs which provide PrismJS are e.g. <a href="https://cdnjs.com/libraries/prism">cdnjs</a>, <a href="https://www.jsdelivr.com/package/npm/prismjs">jsDelivr</a>, and <a href="https://unpkg.com/browse/prismjs@1/">UNPKG</a>.</p>