Update 04-face.md

[joseville1001]

I just tried the exercise and the result of https://portal.vision.cognitive.azure.com/demo/face-detection on store-camera-3.jpg, the picture where the face is almost 100% obscured by a palm frond was "No face detected".

Module: 00

Lab/Demo: 00

Fixes # .

Changes proposed in this pull request:

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[sherzyang]

Thank you @joseville1001!