doc/go_spec: cap doesn't apply to maps

Fixes #4682.

R=golang-dev, adg, dave
CC=golang-dev
https://golang.org/cl/7094062

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of January 18, 2013",
+	"Subtitle": "Version of January 21, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -4922,7 +4922,8 @@ At any time the following relationship holds:
 </pre>
 
 <p>
-The length and capacity of a <code>nil</code> slice, map, or channel are 0.
+The length of a <code>nil</code> slice, map or channel is 0.
+The capacity of a <code>nil</code> slice and channel is 0.
 </p>
 
 <p>