spec: strings are more slices than arrays

Thanks to Aaron Kemp for noticing.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5645097

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 13, 2012"
+	"Subtitle": "Version of February 14, 2012"
 }-->
 
 <!--
@@ -737,7 +737,7 @@ particular architecture.
 
 <p>
 A <i>string type</i> represents the set of string values.
-Strings behave like arrays of bytes but are immutable: once created,
+Strings behave like slices of bytes but are immutable: once created,
 it is impossible to change the contents of a string.
 The predeclared string type is <code>string</code>.