Fix work-dir flag name in example

[ChristophReich1996]

Just fixes a small typo in the example.

[CLAassistant]

All committers have signed the CLA.

[jason102811]

Dear ChristophReich1996,
First of all, we want to express our gratitude for your significant PR in the MMseg project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.
We are looking forward to continuing our collaboration with you. OpenMMLab has established a special contributors' organization called MMSIG, which provides contributors with open-source certificates, a recognition system, and exclusive rewards. You can contact us by adding our WeChat（if you have WeChat): openmmlabwx, or join in our discord： https://discord.gg/qH9fysxPDW. We sincerely hope you will join us!
Best regards！ @ChristophReich1996