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GH-43293: [Docs] Update code block for Installing Java Modules (#43295)



### Rationale for this change

Fix wrong code block formatting in implementations (Java) for Installing Java Modules. 

### What changes are included in this PR?

- Fixed code block.

### Are these changes tested?

Yes. I have verified the changes by building the documentation.

### Are there any user-facing changes?

Yes. the updated documentation will be visible to users.

* GitHub Issue: #43293

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>