examples: fix the image name prefix

[llhuii]

fix the missing sedna-example- prefix image name bug introduced by #214, else our push script would report something:

2021-11-03T10:09:09.0703862Z denied: requested access to the resource is denied
2021-11-03T10:09:09.0724598Z failed to docker push kubeedge/federated-learning-surface-defect-detection-aggregation:v0.4.3
2021-11-03T10:09:09.1243181Z The push refers to repository [docker.io/kubeedge/federated-learning-mistnet-yolo-client]

[JimmyYang20]

/lgtm

[llhuii]

/approve

[kubeedge-bot]

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