doc: simplify Append example in "Effective Go"

Change-Id: I011486993b167e65c69da1c8390bbcc625ca58c3
Reviewed-on: https://go-review.googlesource.com/64331
Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -1431,9 +1431,7 @@ func Append(slice, data []byte) []byte {
         slice = newSlice
     }
     slice = slice[0:l+len(data)]
-    for i, c := range data {
-        slice[l+i] = c
-    }
+    copy(slice[l:], data)
     return slice
 }
 </pre>