Cleanup tutorial document

Summary: made pod coincident with Dist::Milla's as well as possible (and look clean).

Each commit represent:
- Change all shell prompts to `>` (which is also used in Dist::Milla's tutorial)
  - (before this commit, `>` and `%` are jumbled)
- Removed "I don't released yet" phrase.
- Trimed unnecessary white spaces that are absent in Dist::Milla's document.


(Residual diffs: indentations of prompts in Dist::Milla are 2 spaces, while that of this module are 4 spaces. I don't know the reason.)