Correct mistranslation

see: https://github.com/progit/progit2-ja/pull/5#discussion-diff-20081283
Fix mistranslation

see: https://github.com/progit/progit2-ja/pull/5#discussion-diff-20082318
Tweak translation

see: https://github.com/progit/progit2-ja/pull/5#discussion_r20080992