Update SimpleCache examples

[nnlugovoy]

Improve SimpleCache API examples for getOrSet

Current docs state that SimpleCache.getOrSet() returns a SimpleCacheEntry, which is not accurate.
In fact, it returns Promise that resolves to SimpleCacheEntry

Then, example should look like this, with proper await calls

import { SimpleCache } from 'fastly:cache';

addEventListener('fetch', event => event.respondWith(app(event)));

async function app(event) {
  const path = new URL(event.request.url).pathname;
  let page = await SimpleCache.getOrSet(path, async () => {
    return {
      value: await render(path),
      // Store the page in the cache for 1 minute.
      ttl: 60
    }
  });
  if (null === page) {
    return new Response(null, { status: 404 });
  } else {
    let content = await page.text();
    return new Response(content, {
      headers: {
        'content-type': 'text/plain;charset=UTF-8'
      }
    });
  }
}

async function render(path) {
  // expensive/slow function which constructs and returns the contents for a given path
  await new Promise(resolve => setTimeout(resolve, 10_000));
  return path;
}