spec: clarify that shift count must be non-negative

Fixes #34056.

Change-Id: I2c9b7a20d19f458df5dcc376e29bee6be1f09f7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/193277Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Aug 26, 2019",
+	"Subtitle": "Version of Sep 4, 2019",
 	"Path": "/ref/spec"
 }-->
 
@@ -3732,7 +3732,7 @@ be replaced by a bitwise AND operation:
 
 <p>
 The shift operators shift the left operand by the shift count specified by the
-right operand, which must be positive. If the shift count is negative at run time,
+right operand, which must be non-negative. If the shift count is negative at run time,
 a <a href="#Run_time_panics">run-time panic</a> occurs.
 The shift operators implement arithmetic shifts if the left operand is a signed
 integer and logical shifts if it is an unsigned integer.