fix typo (#5667)

packages/vue-server-renderer/README.md

@@ -4,5 +4,5 @@
 
 This package offers Node.js server-side rendering for Vue 2.0.
 
-- [API Reference](ssr.vuejs.org/en/api.html)
+- [API Reference](https://ssr.vuejs.org/en/api.html)
 - [Vue.js Server-Side Rendering Guide](https://ssr.vuejs.org)