[x/blog] go.blog/slices: make slice-header-modifying example actually modify it

Fixes golang/go#6570.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14614043
X-Blog-Commit: 190a69e0573a68c27a4cbda968fadf4eec3c1d60

blog/content/slices/prog020.go

@@ -12,7 +12,8 @@ var buffer [256]byte
 var slice []byte = buffer[100:150]
 
 func SubtractOneFromLength(slice []byte) []byte {
-	return slice[0 : len(slice)-1]
+	slice = slice[0 : len(slice)-1]
+	return slice
 }
 
 func main() {