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Fix typo in Japanese article

First of all, thank you so much for publishing this very educational page.

I found what I believe to be several typos in the Japanese article and have corrected them. A brief explanation is provided below.

• _articles/ja/building-community.md
  • There were two @'s in a row, but one was removed because only one was found on the other language page.
• _articles/ja/how-to-contribute.md, _articles/ja/starting-a-project.md
  • I corrected parts and particles that are unnatural as Japanese when reading.