spec: fix binary expression grammar rule

The spec explains later in the "Operator precedence" section that *
has a higher precedence than +, but the current production rule
requires that "1 + 2 * 3" be parsed as "(1 + 2) * 3", instead of the
intended "1 + (2 * 3)".

The new production rule better matches cmd/internal/gc/go.y's grammar:

    expr:
            uexpr
    |       expr LOROR expr
    |       expr LANDAND expr
    |       ...

Fixes #10151.

Change-Id: I13c9635d6ddf1263cafe7cc63e68f3e5779e24ba
Reviewed-on: https://go-review.googlesource.com/9163Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of March 20, 2015",
+	"Subtitle": "Version of April 22, 2015",
 	"Path": "/ref/spec"
 }-->
 
@@ -3305,7 +3305,7 @@ Operators combine operands into expressions.
 </p>
 
 <pre class="ebnf">
-Expression = UnaryExpr | Expression binary_op UnaryExpr .
+Expression = UnaryExpr | Expression binary_op Expression .
 UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .
 
 binary_op  = "||" | "&amp;&amp;" | rel_op | add_op | mul_op .