Change 'jp' code to 'ja'

Update `Rakefile` to use `ja` rather than `jp` to match renamed Japanese readme (see sinatra/sinatra@ab37432c6a8f3676e8027d76606b370ad2b86662). This was causing an error `No such file or directory @ rb_sysopen - _sinatra/README.jp.md` when trying to build the site.

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Change language code for Japanese readme to 'ja'.
See sinatra/sinatra@ab37432c6a8f3676e8027d76606b370ad2b86662