Unify migration file name formats in "Getting Started with Rails"

Author: wakairo

guides/source/getting_started.md

@@ -1326,7 +1326,7 @@ This command will generate four files:
 
 | File                                         | Purpose                                                                                                |
 | -------------------------------------------- | ------------------------------------------------------------------------------------------------------ |
-| db/migrate/20140120201010_create_comments.rb | Migration to create the comments table in your database (your name will include a different timestamp) |
+| db/migrate/\<timestamp\>_create_comments.rb  | Migration to create the comments table in your database                                                |
 | app/models/comment.rb                        | The Comment model                                                                                      |
 | test/models/comment_test.rb                  | Testing harness for the comment model                                                                 |
 | test/fixtures/comments.yml                   | Sample comments for use in testing                                                                     |