make docker service --help text for --endpoint-mode more consistent
Previously:
```
--constraint value Placement constraints (default [])
--endpoint-mode string Endpoint mode(Valid values: vip, dnsrr)
<snip>
--restart-condition string Restart when condition is met (none, on_failure, or any)
```
Now:
```
--constraint value Placement constraints (default [])
--endpoint-mode string Endpoint mode (vip or dnsrr)
<snip>
--restart-condition string Restart when condition is met (none, on_failure, or any)
```
Signed-off-by: Anil Madhavapeddy <anil@docker.com>