docs: replace references to Skypack CDN with esm.sh (#317)

The Skypack CDN is no longer maintained, so we should remove references
to it.

Author: wolfy1339

README.md

@@ -12,11 +12,11 @@
 <tr><th>
 Browsers
 </th><td width=100%>
-Load <code>@octokit/rest</code> directly from <a href="https://cdn.skypack.dev">cdn.skypack.dev</a>
+Load <code>@octokit/rest</code> directly from <a href="https://esm.sh">esm.sh</a>
 
 ```html
 <script type="module">
-  import { Octokit } from "https://cdn.skypack.dev/@octokit/rest";
+  import { Octokit } from "https://esm.sh/@octokit/rest";
 </script>
 ```