doc: Fix typo in approximation of square root

See https://en.wikipedia.org/wiki/Newton%27s_method#Square_root_of_a_number

R=golang-dev, minux.ma, adg
CC=golang-dev
https://golang.org/cl/9145044

doc/code.html

@@ -295,9 +295,9 @@ package newmath
 
 // Sqrt returns an approximation to the square root of x.
 func Sqrt(x float64) float64 {
-	z := 0.0
+	z := 1.0
 	for i := 0; i < 1000; i++ {
-		z -= (z*z - x) / (2 * x)
+		z -= (z*z - x) / (2 * z)
 	}
 	return z
 }