doc: note go tool tour removal in 1.12 release notes

Note the removal of the go tool tour command in the Go 1.12 release
notes.

Updates #24819

Change-Id: I258ab9401ea2cc06a83328c67299376fcf23c980
Reviewed-on: https://go-review.googlesource.com/c/158618Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.12.html

@@ -113,6 +113,18 @@ go vet -vettool=$(which shadow)
 </pre>
 </p>
 
+<h3 id="tour">Tour</h3>
+
+<p> <!-- CL 152657 -->
+The Go tour is no longer included in the main binary distribution. To
+run the tour locally, instead of running <code>go</code> <code>tool</code> <code>tour</code>,
+manually install it:
+<pre>
+go install golang.org/x/tour
+tour
+</pre>
+</p>
+
 <h3 id="gocache">Build cache requirement</h3>
 
 <p>