Fix bad link for API button in home page (#301)

* fix bad link for API button in home page

* fix spelling typo

Author: ytdHuang

docs/src/index.md

@@ -21,7 +21,7 @@ hero:
       link: https://github.com/qutip/QuantumToolbox.jl
     - theme: alt
       text: API
-      link: /api
+      link: /resources/api
     - theme: alt
       text: Visit QuTiP.org
       link: https://qutip.org/