Update Japanese translation: description about currying

[masmatsum]

Issue Description

Update functional_programming.ja.md about currying, in accordance with ocaml/ocaml.org#1601

Changes Made

• Update functional_programming.ja.md

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• Please check if the PR fulfills these requirements

• PR is descriptively titled and links the original issue above
• Before/after screenshots (if this is a layout change)
• Details of which platforms the change was tested on (if this is a browser-specific change)
• Context for what motivated the change (if this is a change to some content)

[Octachron]

I am not sure if there is a Japanese reviewer around, but this seems essentially correct (through the Google-translate looking glass). I will try to have a look at the French and Italian version.