Update README.md
Change Slack link to Discord link
<a href="https://blog.meilisearch.com">Blog</a> |
<a href="https://docs.meilisearch.com">Documentation</a> |
<a href="https://docs.meilisearch.com/faq/">FAQ</a> |
- <a href="https://slack.meilisearch.com">Slack</a>
+ <a href="https://discord.meilisearch.com">Discord</a>
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