Fix README.ja.md of section 2-2

[RubberGoose246]

In the README.ja.md of the section 2-2, there is a pair of double parentheses enclosing the CSV URL link like below. The double parentheses are causing a 404 error.

このレッスンのために用意された[データ]((../../data/US-pumpkins.csv))を見てみましょう。

[RubberGoose246]

This fix is a very small matter. I just noticed as a Japanese. Also, there is no Japanese translation after the chapter 4. If this fix is not necessary, please delete this request. Anyway, thank you for resourceful contents.

[github-actions[bot]]

This PR has not seen any action for a while! Closing for now, but it can be reopened at a later date.