Add missing link to 2.3.0 docs (#33)

diff --git a/layouts/partials/header.html b/layouts/partials/header.html
index a124f10..1cfce42 100644
--- a/layouts/partials/header.html
+++ b/layouts/partials/header.html
@@ -25,6 +25,7 @@
                     </a>
                     <div class="dropdown-menu" aria-labelledby="ddDocs">
                         <a class="dropdown-item" href="/docs/{{ .Site.Params.latestVersion }}/docs.html">{{ .Site.Params.latestVersion }} (latest)</a>
+                        <a class="dropdown-item" href="/docs/2.3.0/docs.html">2.3.0</a>
                         <a class="dropdown-item" href="/docs/2.2.1/docs.html">2.2.1</a>
                         <a class="dropdown-item" href="/docs/2.1.0/docs.html">2.1.0</a>
                     </div>