content: change variable name in channels example

The code example is summing a slice (s), not an array (a).

Change-Id: Iae4608271a69e1d80fa2c5b09981f5fadc48a3f6
Reviewed-on: https://go-review.googlesource.com/19530
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed