Fixes denoland#4

It seems that the default fresh app screenshot on the README is broken at the moment because the underlying image is missing:

https://deno.com/build-a-blog-with-fresh/default-fresh-app.png

```
curl -LIX GET 'https://deno.com/build-a-blog-with-fresh/default-fresh-app.png' -H "Cache-Control: no-cache"
HTTP/2 404
content-type: text/html; charset=utf-8
cross-origin-embedder-policy: same-origin
cross-origin-opener-policy: same-origin
cross-origin-resource-policy: same-origin
referrer-policy: strict-origin-when-cross-origin
strict-transport-security: max-age=63072000; includeSubDomains; preload
x-content-type-options: nosniff
x-frame-options: DENY
vary: Accept-Encoding
date: Sun, 05 Nov 2023 17:53:47 GMT
via: http/2 edgeproxy-h
server: deno/gcp-us-west4
```

This PR replaces that screenshot path with a GitHub-hosted link.

If it's preferred that the screenshot remains hosted on Deno website (and fixed there), feel free to close this PR.