Updated docs example for redirecting output to enable `docker logs` to work as normal

[andymadge]

Previous example was redirecting both stdout and stderr to PID 1's stdout.

The updated example maintains the separation of stdout and stderr into docker logs, where either stream can be redirected independently as per normal Docker behaviour. To output one of them, you redirect the stream you DON'T want:

Only show stderr: docker logs foo > /dev/null

Only show stdout: docker logs foo 2>/dev/null

Tests not run as this is a docs-only PR.

[funkyfuture]

thank you!