doc: fixed link

Author: ShogunPanda

doc/api/index.md

@@ -7,10 +7,10 @@
 
 * [About this documentation](documentation.md)
 * [Usage and example](synopsis.md)
+* <a href="index.html">Index</a>
 
 <hr class="line"/>
 
-* [Index](index.md)
 * [Assertion testing](assert.md)
 * [Asynchronous context tracking](async_context.md)
 * [Async hooks](async_hooks.md)