[Docs] Update Quick Run Docs

[yeh-sudo]

Motivation

After trying the command in the Quick Run, the pop-up video didn't show up. Furthermore, I found issue#2693, which is similar to my situation. Thus, I decided to resolve it by modifying the document.

Modification

I added an optional --show argument in the detailed document so that the pop-up video will appear after running the command.

The original command in the document:

python demo/demo_inferencer.py  demo/demo.mp4 \
    --rec tsn --print-result \
    --label-file tools/data/kinetics/label_map_k400.txt

After modification:

python demo/demo_inferencer.py  demo/demo.mp4 \
    --rec tsn --print-result --show \
    --label-file tools/data/kinetics/label_map_k400.txt

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