Update README.md (#1199)

link update

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Documentation**
- Updated the URL in the plugins README to direct developers to the
correct introduction page for app integration plugins.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

plugins/README.md

@@ -1,3 +1,3 @@
 # Building Integration Plugins for Our App
 
-https://docs.omi.me/developer/plugins/Introduction/
+https://docs.omi.me/docs/developer/apps/Introduction/