spec: be clearer about the scope of a package name

We have the notion of a PackageName, not package identifier.
As is, it could construed that imports that rename a package
don't have an "imported package identifier" but a local one.

R=r, rsc, iant, ken, dsymonds
CC=golang-dev
https://golang.org/cl/6858049

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of November 17, 2012",
+	"Subtitle": "Version of November 21, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -1529,7 +1529,7 @@ Go is lexically scoped using blocks:
 	    or function (but not method) declared at top level (outside any
 	    function) is the package block.</li>
 
-	<li>The scope of an imported package identifier is the file block
+	<li>The scope of the package name of an imported package is the file block
 	    of the file containing the import declaration.</li>
 
 	<li>The scope of an identifier denoting a function parameter or