Use text_column_name variable instead of "text"
Author: nbroad1881
Pull request: yes

`text_column_name` was already defined above where I made the changes and it was also used below where I made changes.

This is a very minor change. If a dataset does not use "text" as the column name, then the `tokenize_function` will now use whatever column is assigned to `text_column_name`. `text_column_name` is just the first column name if "text" is not a column name. It makes the function a little more robust, though I would assume that 90% + of datasets use "text" anyway.

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