spec: update prose in Constants section

Fixes #5952.

R=r, rsc
CC=golang-dev
https://golang.org/cl/11812043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of July 11, 2013",
+	"Subtitle": "Version of July 25, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -539,7 +539,7 @@ literal.
 <i>rune constants</i>,
 <i>integer constants</i>,
 <i>floating-point constants</i>, <i>complex constants</i>,
-and <i>string constants</i>. Character, integer, floating-point,
+and <i>string constants</i>. Rune, integer, floating-point,
 and complex constants are
 collectively called <i>numeric constants</i>.
 </p>