fix(demo): ensure API docs link is relative

Author: bkrem

demo/src/App.js

@@ -245,7 +245,7 @@ class App extends Component {
                 <h2 className="title">React D3 Tree</h2>
                 <h3 className="title">v{version}</h3>
                 <h3 className="title">
-                  <a href="https://bkrem.github.io/react-d3-tree/docs">📖 API Docs (v2)</a>
+                  <a href="/react-d3-tree/docs">📖 API Docs (v2)</a>
                 </h3>
                 <h4 className="prop">Examples</h4>
                 <div style={{ marginBottom: '5px' }}>