[IMP] emphasize how csv data files should be named

[viso-odoo]

After running the tutorial "Define module data" (under developer/tutorials), I ran into an issue which turned out to be due to bad CSV file naming.

As the error message was not too explicit, I turned towards the web and after a loooong time of searching, finally found my mistake.

In an attempt to save future generations of devs who might go through the same path, I propose the modifications of this commit. All in all, it emphasizes how the csv file should be named.

[robodoo]

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