spec: minor grammar fix

Fixes #20830.

Change-Id: I18178064c955ad8d259df05ee954075ae42909b0
Reviewed-on: https://go-review.googlesource.com/47030Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 7, 2017",
+	"Subtitle": "Version of June 28, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -5025,8 +5025,8 @@ a single byte in the string.
 <li>
 The iteration order over maps is not specified
 and is not guaranteed to be the same from one iteration to the next.
-If map entries that have not yet been reached are removed during iteration,
-the corresponding iteration values will not be produced. If map entries are
+If a map entry that has not yet been reached is removed during iteration,
+the corresponding iteration value will not be produced. If a map entry is
 created during iteration, that entry may be produced during the iteration or
 may be skipped. The choice may vary for each entry created and from one
 iteration to the next.