doc: update install instructions to refer to latest release
<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</code>.
+ It is suggested that you explicitly check out the latest release branch instead, which is currently <code>2020.1.3</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
+git checkout 2020.1.3
go get
go install
</code></pre>
