Fix link to next site (#5428)

Author: mark-wiemer

docs/index.md

@@ -4,7 +4,7 @@ title: 'Mocha - the fun, simple, flexible JavaScript test framework'
 description: 'Mocha is a feature-rich JavaScript test framework running on Node.js and in the browser, making asynchronous testing simple and fun.'
 ---
 
-<div class="admonition">This site has a new look, try it out at <a href="mochajs.org/next">mochajs.org/next</a>!</div>
+<div class="admonition">This site has a new look, try it out at <a href="next">mochajs.org/next</a>!</div>
 
 Mocha is a feature-rich JavaScript test framework running on [Node.js][] and in the browser, making asynchronous testing _simple_ and _fun_. Mocha tests run serially, allowing for flexible and accurate reporting, while mapping uncaught exceptions to the correct test cases. Hosted on [GitHub][github-mocha].