Fix guide about command_line [ci skip]

[yhirano55]

Summary

• Fix description about example generators, when it run bin/rails generate command.
• Insert ones of system test into scaffolding logs.

[rails-bot]

r? @rafaelfranca

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[rafaelfranca]

Why generator was removed? It is there for sure. https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/generator/generator_generator.rb

[yhirano55]

@rafaelfranca I thought it would be better not to change the number of lines. But I noticed it's not make sense, and restored it. Thanks.