spec: clarify meaning of x op= y

Suggested by mdempsky (see also issue #11161).

Change-Id: I1ab28febe19b7a092029499015073ce8749b4d99
Reviewed-on: https://go-review.googlesource.com/10960Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 26, 2015",
+	"Subtitle": "Version of June 11, 2015",
 	"Path": "/ref/spec"
 }-->
 
@@ -4388,7 +4388,7 @@ a[i] = 23
 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
 <code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent
 to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
-<code>y</code> but evaluates <code>x</code>
+<code>(y)</code> but evaluates <code>x</code>
 only once.  The <i>op</i><code>=</code> construct is a single token.
 In assignment operations, both the left- and right-hand expression lists
 must contain exactly one single-valued expression, and the left-hand