doc: update go tour installation instructions

Fixes #16933

Change-Id: I2054abd28bc555b018309934774fc4ecc44826b3
Reviewed-on: https://go-review.googlesource.com/28217Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/docs.html

@@ -40,7 +40,13 @@ The first section covers basic syntax and data structures; the second discusses
 methods and interfaces; and the third introduces Go's concurrency primitives.
 Each section concludes with a few exercises so you can practice what you've
 learned. You can <a href="//tour.golang.org/">take the tour online</a> or
-<a href="//code.google.com/p/go-tour/">install it locally</a>.
+install it locally with:
+</p>
+<p>
+<pre>
+$ go get golang.org/x/tour/gotour
+</pre>
+This will place the <code>gotour</code> binary in your workspace's <code>bin</code> directory.
 </p>
 
 <h3 id="code"><a href="code.html">How to write Go code</a></h3>