Inaccurate example shown in result display for `sort` command.

[nus-se-bot]

Describe the bug When you enter the wrong parameters for the sort command, the example provided in the result display is not a valid command.

To Reproduce Steps to reproduce the behavior:

1. Enter sort.

Expected behavior A valid example such as sort p/expiry d/ascending should be shown.

Screenshots If applicable, add screenshots to help explain your problem.

---

Reported by: @eugeneyl Severity: Low

_{[original: nus-cs2103-AY1819S2/pe-dry-run#298]}

[jtankw3]

Corrected.