content: tweak making-slices example for readability

Adding a bit of whitespace makes the code example more scannable.

Change-Id: Ib8cc8ff2847c3e89d1cd1fcf1e6a0a0aaa562e81
Reviewed-on: https://go-review.googlesource.com/19401
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed