[DOCFIX] Fix tensorflow training script download
[DOCFIX] Fix tensorflow training script download

The original download will download the whole html instead of the
script.

Add `?raw=true` to make it download the python script.

pr-link: Alluxio/alluxio#14763
change-id: cid-792815ca8dfac97387e1e28e9c020b651d563225
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