[WIP] ゲスト講演 「mrubyを使うポイント」のコメント分修正

[kimihito]

65 でもらったコメント分の修正をします。

• [ ] https://github.com/okinawarb/rubima/pull/65#discussion_r11398506
• [x] https://github.com/okinawarb/rubima/pull/65#discussion_r11398600
• [x] https://github.com/okinawarb/rubima/pull/65#discussion_r11398618
• [x] https://github.com/okinawarb/rubima/pull/65#discussion_r11399138
• [ ] https://github.com/okinawarb/rubima/pull/65#discussion_r11398775
• [x] https://github.com/okinawarb/rubima/pull/65#discussion_r11398830
• [ ] https://github.com/okinawarb/rubima/pull/65#discussion_r11399005
• [x] https://github.com/okinawarb/rubima/pull/65#discussion_r11401952