fix case studies link on README (#19543)

Automatic Merge

README.md

@@ -70,7 +70,7 @@ Receive notifications of critical security updates. The sophistication of online
 ## Learn more
 
 - [API options - webhooks, slash commands, drivers, and web service](https://api.mattermost.com/)
-- [See who's using Mattermost](https://mattermost.com/blog/category/customers/)
+- [See who's using Mattermost](https://mattermost.com/customers/)
 - [Browse over 700 Mattermost integrations](https://mattermost.com/marketplace/)
 
 ## License