fix: Correct link for mailing list in landing page

[jpmcb]

Fixes #195

---

This fixes the broken link in the website landing page for the mailing lists: I found the correct mailing list link in this repo's README.md

---

Before this change:

❯ rg https://lists.cncf.io/g/cncf-runtime/topics
website/content/en/_index.md
29:- [Mailing list](https://lists.cncf.io/g/cncf-runtime/topics)

website/content/ja/_index.md
28:- [メーリングリスト](https://lists.cncf.io/g/cncf-runtime/topics)

After this change:

❯ rg https://lists.cncf.io/g/cncf-runtime/topics

[netlify[bot]]

✅ Deploy Preview for tag-runtime ready!

Name	Link
🔨 Latest commit	7f6168d2cfa7e2d1863f2271df4e451ca0460ff5
🔍 Latest deploy log	https://app.netlify.com/sites/tag-runtime/deploys/673e5df1cc13ac00085f4e5b
😎 Deploy Preview	https://deploy-preview-196--tag-runtime.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[stmcginnis]

/lgtm

:tada: