doc/articles: state that slices/arrays start at the 0th element

Fixes #5881.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11892043

doc/articles/slices_usage_and_internals.html

@@ -25,7 +25,7 @@ the type <code>[4]int</code> represents an array of four integers. An array's
 size is fixed; its length is part of its type (<code>[4]int</code> and
 <code>[5]int</code> are distinct, incompatible types). Arrays can be indexed in
 the usual way, so the expression <code>s[n]</code> accesses the <i>n</i>th
-element:
+element, starting from zero.
 </p>
 
 <pre>