Alter the README.md in pose_estimation/raspberry_pi

[ChenBingWei1201]

I executed the TensorFlow pose_estimation.py on my raspberry pi 4 last weekend. However, an error popped out when I added --model_name movenet_multipose behind pose_estimation.py. It said --model model_name. After reviewing the code in pose_estimation.py(line 161), I finally realized that the error is --model_name. Additionally, a similar problem occurred when I added classifier classifier behind the file. It is classifier classifier.tflite actually. Therefore, I want to replace --model_name with --model and classifier classifier with classifier classifier.tflite so that maybe it will be more user-friendly.