Fix video extraction command.

[rosenfeldamir]

The command cat 20bn-something-something-v2-?? | tar zx did not work for me directly. The 20 zip files must be first unzipped and only then can they be concatenated for tar to extract.

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[kennymckormick]

Would you please also update the links for downloading the something-something videos (both v1 and v2), since the old links are invalid now.

[kennymckormick]

Also, CLA needs to be signed.

[OpenMMLab-Assistant-004]

Hi @rosenfeldamir ！We are grateful for your efforts in helping improve mmaction2 open-source project during your personal time.

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