Fix the Japanese tranlation of items_per_page.

[bandaot]

Like the Chinese translation, it's proper to add a '件' here.

[vercel]

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[codecov]

Codecov Report

Merging #334 (65da986) into master (c1901f9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #334   +/-   ##
=======================================
  Coverage   92.35%   92.35%           
=======================================
  Files           3        3           
  Lines         327      327           
  Branches      113      113           
=======================================
  Hits          302      302           
  Misses         25       25           

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