spec: fix small typo in comment for example

R=golang-dev, mirtchovski, r
CC=golang-dev
https://golang.org/cl/14227043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Sep 16, 2013",
+	"Subtitle": "Version of Oct 3, 2013",
 	"Path": "/doc/spec"
 }-->
 
@@ -2671,7 +2671,7 @@ sliced operand:
 </p>
 
 <pre>
-a[2:]  // same a[2 : len(a)]
+a[2:]  // same as a[2 : len(a)]
 a[:3]  // same as a[0 : 3]
 a[:]   // same as a[0 : len(a)]
 </pre>