Fix a link in changelog

Author: seratch

docs-src/changelog.rst

@@ -5,7 +5,7 @@ Changelog
 v2.6.0rc1 (2020-05-14)
 -------------------
 
-Refer to [v2.6.0 milestone](https://github.com/slackapi/python-slackclient/milestone/5) to know the complete list of the issues resolved by this release.
+Refer to `v2.6.0 milestone <https://github.com/slackapi/python-slackclient/milestone/5>`_ to know the complete list of the issues resolved by this release.
 
 **New Features**
 

docs/changelog.html

@@ -225,7 +225,7 @@
 <h1>Changelog<a class="headerlink" href="#changelog" title="Permalink to this headline">¶</a></h1>
 <div class="section" id="v2-6-0rc1-2020-05-14">
 <h2>v2.6.0rc1 (2020-05-14)<a class="headerlink" href="#v2-6-0rc1-2020-05-14" title="Permalink to this headline">¶</a></h2>
-<p>Refer to [v2.6.0 milestone](<a class="reference external" href="https://github.com/slackapi/python-slackclient/milestone/5">https://github.com/slackapi/python-slackclient/milestone/5</a>) to know the complete list of the issues resolved by this release.</p>
+<p>Refer to <a class="reference external" href="https://github.com/slackapi/python-slackclient/milestone/5">v2.6.0 milestone</a> to know the complete list of the issues resolved by this release.</p>
 <p><strong>New Features</strong></p>
 <ol class="arabic simple">
 <li><p>[Block Kit] #659 Add complete supports for Block Kit components and fixed a few existing bugs as well (#500 #519 #623 #632 #635 #639 #676) - Thanks &#64;seratch &#64;diurnalist &#64;ruberVulpes &#64;jeremyschulman &#64;e271828- &#64;RodneyU215</p></li>