spec: Use "non-negative" instead of "positive"

Replacing division-by-power-of-2 with right-shift is valid for
zero too.

R=gri
CC=golang-dev
https://golang.org/cl/7027049

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of January 2, 2013",
+	"Subtitle": "Version of January 6, 2013",
 	"Path": "/ref/spec"
 }-->
 
@@ -3027,7 +3027,7 @@ int64    -9223372036854775808
 <p>
 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
-If the dividend is positive and the divisor is a constant power of 2,
+If the dividend is non-negative and the divisor is a constant power of 2,
 the division may be replaced by a right shift, and computing the remainder may
 be replaced by a bitwise AND operation:
 </p>