[Doc] Fix API doc link in side navigation (#22585)

Signed-off-by: 22quinn <33176974+22quinn@users.noreply.github.com>

docs/.nav.yml

@@ -11,7 +11,7 @@ nav:
     - Quick Links:
       - User Guide: usage/README.md
       - Developer Guide: contributing/README.md
-      - API Reference: api/README.md
+      - API Reference: api/summary.md
       - CLI Reference: cli/README.md
     - Timeline:
       - Roadmap: https://roadmap.vllm.ai