fix(caching-guide): correct jokeAPI.get() signature in complete example (#521)

The 'Putting It All Together' example used `async get()` with `this.getId()`
which throws at runtime. The correct signature is `async get(id)` — consistent
with the earlier 'Wrapping an External Data Source' section and the working
caching-guide-example repo.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kriszyp

learn/developers/caching-with-harper.mdx

@@ -335,8 +335,7 @@ Here is the complete `resources.js` for this guide:
 // resources.js
 
 const jokeAPI = {
-	async get() {
-		const id = this.getId();
+	async get(id) {
 		const response = await fetch(`https://official-joke-api.appspot.com/jokes/${id}`);
 		return response.json();
 	},