readme: fix broken api ref link (#452)

Author: psugihara

README.md

@@ -4,7 +4,7 @@
 </p>
 
 <p align="center">
-  <a href="https://nitro.jan.ai/docs">Documentation</a> - <a href="https://nitro.jan.ai/api">API Reference</a> 
+  <a href="https://nitro.jan.ai/docs">Documentation</a> - <a href="https://nitro.jan.ai/api-reference">API Reference</a> 
   - <a href="https://github.com/janhq/nitro/releases/">Changelog</a> - <a href="https://github.com/janhq/nitro/issues">Bug reports</a> - <a href="https://discord.gg/AsJ8krTT3N">Discord</a>
 </p>