MINOR: Update docs to say 2.2 (#6315)

Update docs to say 2.2

Reviewers: Jason Gustafson <jason@confluent.io>, Bill Bejeck <bbejeck@gmail.com>

docs/documentation.html

@@ -26,8 +26,8 @@
 	<div class="right">
 		<!--#include virtual="../includes/_docs_banner.htm" -->
     <h1>Documentation</h1>
-    <h3>Kafka 2.1 Documentation</h3>
-    Prior releases: <a href="/07/documentation.html">0.7.x</a>, <a href="/08/documentation.html">0.8.0</a>, <a href="/081/documentation.html">0.8.1.X</a>, <a href="/082/documentation.html">0.8.2.X</a>, <a href="/090/documentation.html">0.9.0.X</a>, <a href="/0100/documentation.html">0.10.0.X</a>, <a href="/0101/documentation.html">0.10.1.X</a>, <a href="/0102/documentation.html">0.10.2.X</a>, <a href="/0110/documentation.html">0.11.0.X</a>, <a href="/10/documentation.html">1.0.X</a>, <a href="/11/documentation.html">1.1.X</a>, <a href="/20/documentation.html">2.0.X</a>.
+    <h3>Kafka 2.2 Documentation</h3>
+    Prior releases: <a href="/07/documentation.html">0.7.x</a>, <a href="/08/documentation.html">0.8.0</a>, <a href="/081/documentation.html">0.8.1.X</a>, <a href="/082/documentation.html">0.8.2.X</a>, <a href="/090/documentation.html">0.9.0.X</a>, <a href="/0100/documentation.html">0.10.0.X</a>, <a href="/0101/documentation.html">0.10.1.X</a>, <a href="/0102/documentation.html">0.10.2.X</a>, <a href="/0110/documentation.html">0.11.0.X</a>, <a href="/10/documentation.html">1.0.X</a>, <a href="/11/documentation.html">1.1.X</a>, <a href="/20/documentation.html">2.0.X</a>, <a href="/21/documentation.html">2.1.X</a>.
 
     <!--#include virtual="toc.html" -->