doc: clarify package uninstall/clean command

Fixes golang#24864

Change-Id: Iaffd23b368aff663bfc09a4e6277c01e7b5ea48d
GitHub-Last-Rev: 98e39b7
GitHub-Pull-Request: golang#24865
Reviewed-on: https://go-review.googlesource.com/106986
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Rob Pike <r@golang.org>

doc/install.html

@@ -286,7 +286,7 @@ <h2 id="testing">Test your installation</h2>
 <p>
 You can run <code>go</code> <code>install</code> to install the binary into
 your workspace's <code>bin</code> directory
-or <code>go</code> <code>clean</code> to remove it.
+or <code>go</code> <code>clean</code> <code>-i</code> to remove it.
 </p>
 
 <p>