docs: update knative-sandbox references to knative-extensions

[Kunal1522]

What this PR does

Updates outdated references from knative-sandbox to knative-extensions in the "Getting Started in Open Source with Knative Part 2" blog post.
The knative-sandbox GitHub organization was renamed to knative-extensions as part of Knative's organizational restructuring.

Changes

File: [docs/blog/articles/getting-started-blog-p2.md](https://github.com/knative/docs/blob/main/docs/blog/articles/getting-started-blog-p2.md)

• Line 257: Updated organization name in the context of running tests on PRs
• Line 259: Updated organization name in the context of joining the organization

Why this matters

New contributors reading this blog post may be confused by references to an organization that no longer exists under that name.

Related

• knative-extensions organization
  /kind documentation

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