Renames japanese doc files

[ikenami]

Summary

Renames doc files that started with "jp_" to "ja_" in order to match the other existing japanese doc files.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #273 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   59.95%   59.95%           
=======================================
  Files           7        7           
  Lines         492      492           
  Branches      136      136           
=======================================
  Hits          295      295           
  Misses        169      169           
  Partials       28       28

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[seratch]

Good catch, LGTM 👍 (just in case, I've verified if the changes work)