Add gdown to requirements.txt

[mrpeerat]

Title: Add gdown to requirements.txt (we forgot to put it to the list)

First line PR Message: I install seacrowd using python 3.10, and when I ran it, the message said the gdown package was missing.

[ljvmiranda921]

Hi there @mrpeerat , hmm, there's already gdown in Line 9 of the requirements.txt file. That's weird 🤔 Did you by chance install it in a different environment?

[mrpeerat]

I see. I used pip install seacrowd, and I guess we might have forgotten to update the list in lib.