Small doc update for exec command

The documentation for `-tag` says it must be used with `-service`, but the example shown is `-server`, which doesn't work (probably a typo or damn-you-autocorrect):

```console
$ consul exec -server web -tag rails uptime
flag provided but not defined: -server
Usage: consul exec [options] [-|command...]
```

This PR changes `-server` to `-service`:

```console
$ consul exec -service web -tag rails uptime
    agent-one:  04:48:53 up 36 min,  2 users,  load average: 0.16, 0.35, 0.29
    agent-one:
==> agent-one: finished with exit code 0
1 / 1 node(s) completed / acknowledged
```