Change the plot script location of detect_faces_mtcnn

Edited the symbolic link to point to bob.ip.facedetect instead of bob.ip.tensorflow_extractor. (related to bob.ip.facedetect!13 (merged))

doc/plot/detect_faces_mtcnn.py

-../bob/bob.ip.tensorflow_extractor/doc/plot/detect_faces_mtcnn.py
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+../bob/bob.ip.facedetect/doc/plot/detect_faces_mtcnn.py
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