[x/blog] go.blog/slices: fix bug in Append

The code didn't work but the blog runs the previous simple version, which does,
so when you hit Run it looks like it's working.
It only had two bugs (so far).

Thanks to shelah.kell@gmail.com for noticing.

R=adg
CC=golang-dev
https://golang.org/cl/14036043
X-Blog-Commit: d4c23261dc940eb2cb841a1a84832174e6bac0c4

blog/content/slices.article

@@ -443,7 +443,7 @@ allocated memory, and then to copy the appending items to the end of the old dat
 
 Try it; the behavior is the same as before:
 
-.play -edit slices/prog130.go /START/,/END/
+.play -edit slices/prog140.go /START/,/END/
 
 * Append: The built-in function
 

blog/content/slices/prog140.go

@@ -11,7 +11,8 @@ import (
 // Append appends the elements to the slice.
 // Efficient version.
 func Append(slice []int, elements ...int) []int {
-	total := len(slice) + len(items)
+	n := len(slice)
+	total := len(slice) + len(elements)
 	if total > cap(slice) {
 		// Reallocate. Grow to 1.5 times the new size, so we can still grow.
 		newSize := total*3/2 + 1
@@ -19,9 +20,8 @@ func Append(slice []int, elements ...int) []int {
 		copy(newSlice, slice)
 		slice = newSlice
 	}
-	n := len(slice)
 	slice = slice[:total]
-	copy(slice[:n], elements)
+	copy(slice[n:], elements)
 	return slice
 }