test/ken/slicearray.go: correct type width in comment

The type was changed in https://golang.org/cl/3991043 but the comment
wasn't updated.

Change-Id: I7ba3f625c732e5e801675ffc5d4a28e1d310faa3
Reviewed-on: https://go-review.googlesource.com/c/go/+/369374
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

Author: ianlancetaylor

test/ken/slicearray.go

@@ -89,7 +89,7 @@ func main() {
 	by = bx[2:8]
 	tstb()
 
-	// width 4 (float64)
+	// width 8 (float64)
 	lb = 0
 	hb = 10
 	fy = fx[lb:hb]