doc/go1.2.html: fix 3-index slice example

A number was wrong; adjust as suggested to make things clearer.
Fixes #6452

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/13422046

doc/go1.2.html

@@ -99,12 +99,12 @@ source slice or array, adjusted for the origin. For instance,
 </p>
 
 <pre>
-slice = array[2:4:6]
+slice = array[2:4:7]
 </pre>
 
 <p>
-sets the slice to have the same length as in the earlier example but its capacity is now only 4 elements (6-2).
-It is impossible to use this new slice value to access the last two elements of the original array.
+sets the slice to have the same length as in the earlier example but its capacity is now only 5 elements (7-2).
+It is impossible to use this new slice value to access the last three elements of the original array.
 </p>
 
 <p>