Unify migration file name formats in "Getting Started with Rails" [ci skip]

[wakairo]

Motivation / Background

This Pull Request has been created because there are two different migration file name formats as below.

• db/migrate/<timestamp>_create_articles.rb
• db/migrate/20140120201010_create_comments.rb with a comment: "your name will include a different timestamp"

Detail

This Pull Request unifies the format as below.

• db/migrate/<timestamp>_create_articles.rb
• db/migrate/<timestamp>_create_comments.rb

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[skipkayhil]

Thanks!