doc: the latest version is 2020.1.6

Author: dominikh

doc/staticcheck.html

@@ -22,12 +22,12 @@ <h2 id="installation">Installation</h2>
 <p>
   If you use Go modules, you can simply run <code>go get honnef.co/go/tools/cmd/staticcheck</code> to obtain the latest released version.
   If you're still using a GOPATH-based workflow, then the above command will instead fetch the master branch.
-  It is suggested that you explicitly check out the latest release branch instead, which is currently <code>2020.1.5</code>.
+  It is suggested that you explicitly check out the latest release tag instead, which is currently <code>2020.1.6</code>.
   One way of doing so would be as follows:
 </p>
 
 <pre><code>cd $GOPATH/src/honnef.co/go/tools/cmd/staticcheck
-git checkout 2020.1.4
+git checkout 2020.1.6
 go get
 go install
 </code></pre>