spec: delete spurious article

A profound change to christen the new tag.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5327062

doc/go_spec.html

@@ -763,7 +763,7 @@ The length is part of the array's type and must be a
 integer value.  The length of array <code>a</code> can be discovered
 using the built-in function <a href="#Length_and_capacity"><code>len(a)</code></a>.
 The elements can be indexed by integer
-indices 0 through the <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>).
+indices 0 through <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>).
 Array types are always one-dimensional but may be composed to form
 multi-dimensional types.
 </p>